Compile with _GNU_SOURCE when appropriate

On non-Windows builds, we will now use _GNU_SOURCE so header
files will include modern API extensions.  This should resolve
issue #547.

CMakeLists.txt

@@ -65,7 +65,7 @@ IF (MSVC)
 	SET(CMAKE_C_FLAGS_RELEASE "/MT /O2")
 	SET(WIN_RC "src/win32/git2.rc")
 ELSE ()
-	SET(CMAKE_C_FLAGS "-O2 -g -Wall -Wextra -Wno-missing-field-initializers -Wstrict-aliasing=2 -Wstrict-prototypes -Wmissing-prototypes ${CMAKE_C_FLAGS}")
+	SET(CMAKE_C_FLAGS "-O2 -g -D_GNU_SOURCE -Wall -Wextra -Wno-missing-field-initializers -Wstrict-aliasing=2 -Wstrict-prototypes -Wmissing-prototypes ${CMAKE_C_FLAGS}")
 	SET(CMAKE_C_FLAGS_DEBUG "-O0 -g")
 	IF (NOT MINGW) # MinGW always does PIC and complains if we tell it to
 		SET(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -fPIC")