package/libgdiplus: needs C++

Unfortunately, libgdiplus unconditionally calls AC_PROG_CXX since
version 6.1 for google-based tests resulting in the following build
failure without C++ since commit
5b6dd17b864cfb9133d3e2d78b9a15fed877b15b and
https://github.com/mono/libgdiplus/commit/4f9802230653ffe810ed2e535a572e42821fdf2d:

checking whether the C++ compiler works... no
configure: error: in `/home/thomas/autobuild/instance-3/output-1/build/libgdiplus-6.1':
configure: error: C++ compiler cannot create executables

Fixes:
 - http://autobuild.buildroot.org/results/3757921a2160ca209089a0b47414a445cc42e35e

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/libgdiplus/Config.in

@@ -4,6 +4,7 @@ config BR2_PACKAGE_LIBGDIPLUS
 	depends on BR2_USE_WCHAR # libglib2 -> gettext
 	depends on BR2_TOOLCHAIN_HAS_THREADS # libglib2
 	depends on BR2_USE_MMU # libglib2
+	depends on BR2_INSTALL_LIBSTDCPP
 	select BR2_PACKAGE_XLIB_LIBXFT
 	select BR2_PACKAGE_LIBGLIB2
 	select BR2_PACKAGE_CAIRO
@@ -13,7 +14,8 @@ config BR2_PACKAGE_LIBGDIPLUS
 
 	  https://github.com/mono/libgdiplus
 
-comment "libgdiplus needs a toolchain w/ wchar, threads"
+comment "libgdiplus needs a toolchain w/ wchar, threads, C++"
 	depends on BR2_USE_MMU
 	depends on BR2_PACKAGE_XORG7
-	depends on !BR2_USE_WCHAR || !BR2_TOOLCHAIN_HAS_THREADS
+	depends on !BR2_USE_WCHAR || !BR2_TOOLCHAIN_HAS_THREADS || \
+		!BR2_INSTALL_LIBSTDCPP