cmake: Use vendored libgit2 by default

Just because build system has libgit2 headers globally, doesn't mean we
need to use them by default :/

Use LIBGIT2_USE_VENDORED=OFF to override this and look for installed
package.

Author: craigds

CMakeLists.txt

@@ -5,13 +5,15 @@ SET(CMAKE_ARCHIVE_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/lib)
 SET(CMAKE_LIBRARY_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/lib)
 
 # Add libgit2
-find_package(PkgConfig)
-if(PkgConfig_FOUND)
-  PKG_CHECK_MODULES(LIBGIT2 libgit2>=1.4.0)
-endif()
-if(NOT LIBGIT2_FOUND)
+OPTION(LIBGIT2_USE_VENDORED "Use the vendored libgit2, instead of finding the package externally" ON)
+if(LIBGIT2_USE_VENDORED)
   ADD_SUBDIRECTORY(ext/libgit2 ${CMAKE_BINARY_DIR}/lib)
   set(LIBGIT2_INCLUDEDIR ext/libgit2/include)
+else()
+  find_package(PkgConfig)
+  if(PkgConfig_FOUND)
+    PKG_CHECK_MODULES(LIBGIT2 libgit2>=1.4.0)
+  endif()
 endif()
 
 INCLUDE(CMakePackageConfigHelpers)