First draft for Android cross-compilation support

Adds options to `configure` in a similar fashion and with similar
constraints to the options added in `swift` to add Android Support.
This will allow initially those features to be just passed from
`utils/build-script` without any change until a full strategy for
cross-compilation in all the involved projects is defined.

This is an example call to build for Android with Swift support:
```
env \
CC="${swift_android_path}/build/Ninja-ReleaseAssert/llvm-linux-x86_64/bi
n/clang" \
CXX="${swift_android_path}/build/Ninja-ReleaseAssert/llvm-linux-x86_64/b
in/clang++" \
SWIFTC="${swift_android_path}/build/Ninja-ReleaseAssert/swift-linux-x86_
64/bin/swiftc" \
${swift_android_path}/swift-corelibs-libdispatch/configure \
--with-swift-toolchain=“${swift_android_path}/build/Ninja-ReleaseAssert/
swift-linux-x86_64/" \
--with-build-variant=release \
--enable-android \
--host=arm-linux-androideabi \
--with-android-ndk=${ndk_path} \
--with-android-api-level=21 \
--disable-build-tests
```

* NOTE: `shims/android/lib(pthread|rt).a` were added to avoid modifying
`libpwq` and `libkqueue` submodules. This two projects are adding
`-lpthread -lrt` in `dependency_libs` at their libtool `la` files,
causing this to be extended until the final linking of the dynamic
library. There should be a more elegant solution to this issue.

Author: gonzalolarralde

configure.ac

@@ -11,6 +11,10 @@ AC_CONFIG_MACRO_DIR([m4])
 ac_clean_files=a.out.dSYM
 AM_MAINTAINER_MODE
 
+AC_CANONICAL_BUILD
+AC_CANONICAL_HOST
+AC_CANONICAL_TARGET
+
 #
 # Command line argument to specify build variant (default to release).
 # Impacts default value of CFLAGS et al. so must come before AC_PROG_CC
@@ -56,6 +60,53 @@ AC_PROG_CXX([clang++ g++ c++])
 AC_PROG_OBJC([clang gcc cc])
 AC_PROG_OBJCXX([clang++ g++ c++])
 
+#
+# Android cross-compilation support
+#
+AC_ARG_WITH([android-ndk],
+  [AS_HELP_STRING([--with-android-ndk],
+    [Android NDK location])], [
+  android_ndk=${withval}
+])
+AC_ARG_WITH([android-ndk-gcc-version],
+  [AS_HELP_STRING([--with-android-ndk-gcc-version],
+    [Android NDK GCC version [defaults=4.9]])],
+  [android_ndk_gcc_version=${withval}], [android_ndk_gcc_version=4.9])
+AC_ARG_WITH([android-api-level],
+  [AS_HELP_STRING([--with-android-api-level],
+    [Android API level to link with])], [
+  android_api_level=${withval}
+])
+AC_ARG_ENABLE([android],
+  [AS_HELP_STRING([--enable-android],
+    [Compile for Android])], [
+  android=true
+
+  # Override values until there's real support for multiple Android platforms
+  host=armv7-none-linux-androideabi
+  host_alias=arm-linux-androideabi
+  host_cpu=armv7
+  host_os=linux-androideabi
+  host_vendor=unknown
+  arch=arm
+
+  sysroot=${android_ndk}/platforms/android-${android_api_level}/arch-${arch}
+  toolchain=${android_ndk}/toolchains/${host_alias}-${android_ndk_gcc_version}/prebuilt/linux-${build_cpu}
+
+  CFLAGS="$CFLAGS -target ${host_alias} --sysroot=${sysroot} -B${toolchain}/${host_alias}/bin"
+  CXXFLAGS="$CXXFLAGS -target ${host_alias} --sysroot=${sysroot} -B${toolchain}/${host_alias}/bin"
+  SWIFTC_FLAGS="-target ${host} -sdk ${sysroot} -L${toolchain}/lib/gcc/${host_alias}/${android_ndk_gcc_version}.x"
+  LIBS="$LIBS -L${toolchain}/lib/gcc/${host_alias}/${android_ndk_gcc_version}.x"
+  LDFLAGS="$LDFLAGS -Wc,'-target','${host_alias}','-B${toolchain}/${host_alias}/bin'"
+
+  # FIXME: empty CFLAGS and CXXFLAGS are assumed for this to work.
+  # FIXME: there should be a more elegant way to do this
+  ac_configure_args=`echo $ac_configure_args | sed -e "s/ 'CFLAGS='//" -e "s/ 'CXXFLAGS='//"`
+  # CFLAGS, CXXFLAGS and LIBS needs to be passed to libkqueue and libpwq
+  ac_configure_args="$ac_configure_args 'CFLAGS=$CFLAGS' 'CXXFLAGS=$CXXFLAGS' 'LIBS=$LIBS'"
+], [android=false])
+AM_CONDITIONAL(ANDROID, $android)
+
 #
 # On Mac OS X, some required header files come from other source packages;
 # allow specifying where those are.
@@ -134,8 +185,6 @@ AS_IF([test "x$enable_apple_tsd_optimizations" = "xyes"],
     [Define to use non-portable pthread TSD optimizations for Mac OS X)])]
 )
 
-AC_CANONICAL_TARGET
-
 #
 # Enable building Swift overlay support into libdispatch
 #

src/Makefile.am

@@ -64,10 +64,17 @@ DISPATCH_CFLAGS=-Wall $(VISIBILITY_FLAGS) $(OMIT_LEAF_FP_FLAGS) \
 if DISPATCH_ENABLE_ASSERTS
 DISPATCH_CFLAGS+=-DDISPATCH_DEBUG=1
 endif
-AM_CFLAGS= $(KQUEUE_CFLAGS) $(PTHREAD_WORKQUEUE_CFLAGS) $(DISPATCH_CFLAGS) $(CBLOCKS_FLAGS)
-AM_OBJCFLAGS=$(DISPATCH_CFLAGS) $(CBLOCKS_FLAGS)
-AM_CXXFLAGS=$(KQUEUE_CFLAGS) $(PTHREAD_WORKQUEUE_CFLAGS) $(DISPATCH_CFLAGS) $(CXXBLOCKS_FLAGS)
-AM_OBJCXXFLAGS=$(DISPATCH_CFLAGS) $(CXXBLOCKS_FLAGS)
+
+if ANDROID
+CANDROID_FLAGS= -I$(top_srcdir)/src/shims/android -L$(top_srcdir)/src/shims/android
+CXXANDROID_FLAGS= -I$(top_srcdir)/src/shims/android -L$(top_srcdir)/src/shims/android
+ANDROID_LIBS= -L$(top_srcdir)/src/shims/android
+endif
+
+AM_CFLAGS= $(KQUEUE_CFLAGS) $(PTHREAD_WORKQUEUE_CFLAGS) $(DISPATCH_CFLAGS) $(CBLOCKS_FLAGS) $(CANDROID_FLAGS)
+AM_OBJCFLAGS=$(DISPATCH_CFLAGS) $(CBLOCKS_FLAGS) $(CANDROID_FLAGS)
+AM_CXXFLAGS=$(KQUEUE_CFLAGS) $(PTHREAD_WORKQUEUE_CFLAGS) $(DISPATCH_CFLAGS) $(CXXBLOCKS_FLAGS) $(CXXANDROID_FLAGS)
+AM_OBJCXXFLAGS=$(DISPATCH_CFLAGS) $(CXXBLOCKS_FLAGS) $(CXXANDROID_FLAGS)
 
 if BUILD_OWN_KQUEUES
   KQUEUE_LIBS+=$(top_builddir)/libkqueue/libkqueue.la
@@ -91,7 +98,7 @@ BLOCKS_RUNTIME_LIBS=-ldl
 endif
 
 libdispatch_la_LDFLAGS=-avoid-version
-libdispatch_la_LIBADD=$(KQUEUE_LIBS) $(PTHREAD_WORKQUEUE_LIBS) $(BSD_OVERLAY_LIBS) $(BLOCKS_RUNTIME_LIBS)
+libdispatch_la_LIBADD=$(KQUEUE_LIBS) $(PTHREAD_WORKQUEUE_LIBS) $(BSD_OVERLAY_LIBS) $(BLOCKS_RUNTIME_LIBS) $(ANDROID_LIBS)
 
 if HAVE_DARWIN_LD
 libdispatch_la_LDFLAGS+=-Wl,-compatibility_version,1 \

src/shims/android/libpthread.a

@@ -0,0 +1 @@
+!<arch>

src/shims/android/librt.a

@@ -0,0 +1 @@
+!<arch>