BUILD: Attempt to fix Android builds when building the test runner

Android builds currently fail when building the test runner because
the runner tries to include standard library headers, which cannot
be found. This patch adds the gcc-stdlibc++ include directory that
matches the existing gcc-stdlibc++ linker flag.

configure

@@ -2358,6 +2358,7 @@ case $_host_os in
 			append_var LDFLAGS "-mthumb-interwork"
 		fi
 
+		append_var CXXFLAGS "-isystem $ANDROID_NDK/sources/cxx-stl/gnu-libstdc++/`$CXX -dumpversion`/include/"
 		append_var LDFLAGS "-L$ANDROID_NDK/sources/cxx-stl/gnu-libstdc++/`$CXX -dumpversion`/libs/$ABI/"
 		append_var LIBS "-lsupc++"
 		add_line_to_config_mk "ANDROID_SDK = $ANDROID_SDK"