Add comment on symbol searching

Add comment describing why we need to check for the availability of
strlcat/strlcpy and issetguid.

Also removed setting the _GNU_SOURCES compile-definition and
-fblocks flag as it's already added through one of the global variables.

Author: etcwilde

CMakeLists.txt

@@ -99,6 +99,12 @@ include(CheckSymbolExists)
 
 check_linker_flag(C "LINKER:--build-id=sha1" LINKER_SUPPORTS_BUILD_ID)
 
+# Detect if the system libc defines symbols for these functions.
+# If it is not availble, swift-corelibs-foundation has its own implementations
+# that will be used. If it is available, it should not redefine them.
+# Note: SwiftPM does not have the ability to introspect the contents of the SDK
+#       and therefore will always include these functions in the build and will
+#       cause build failures on platforms that define these functions.
 check_symbol_exists("strlcat" "string.h" HAVE_STRLCAT)
 check_symbol_exists("strlcpy" "string.h" HAVE_STRLCPY)
 check_symbol_exists("issetugid" "unistd.h" HAVE_ISSETUGID)
@@ -107,17 +113,11 @@ add_compile_definitions(
   $<$<AND:$<COMPILE_LANGUAGE:C>,$<BOOL:${HAVE_STRLCPY}>>:HAVE_STRLCPY>
   $<$<AND:$<COMPILE_LANGUAGE:C>,$<BOOL:${HAVE_ISSETUGID}>>:HAVE_ISSETUGID>)
 
-if(CMAKE_SYSTEM_NAME STREQUAL Linux OR ANDROID)
-  add_compile_definitions($<$<COMPILE_LANGUAGE:C>:_GNU_SOURCE>)
-endif()
-
 if(CMAKE_SYSTEM_NAME STREQUAL Linux)
   check_symbol_exists(sched_getaffinity "sched.h" HAVE_SCHED_GETAFFINITY)
   add_compile_definitions($<$<COMPILE_LANGUAGE:C>:HAVE_SCHED_GETAFFINITY>)
 endif()
 
-add_compile_options($<$<COMPILE_LANGUAGE:C>:-fblocks>)
-
 # Precompute module triple for installation
 if(NOT SwiftFoundation_MODULE_TRIPLE)
     set(module_triple_command "${CMAKE_Swift_COMPILER}" -print-target-info)