Fix autoconf>=2.70 gcc-wrapper breakage

Re-generated configure file gets confused & claims not finding C89-
compatible compiler for gcc-wrapper-building libassuan with msvc
underneath.

Work-around the problem by telling toolchain right off that this
_is_ a std c compliant compiler.

Change-Id: I4fa23673b790bc70a9294951df545c27f5236f81
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/127641
Tested-by: Jenkins
Reviewed-by: Thorsten Behrens <thorsten.behrens@allotropia.de>
diff --git a/external/libassuan/ExternalProject_libassuan.mk b/external/libassuan/ExternalProject_libassuan.mk
index 8ee9682..a4c2510 100644
--- a/external/libassuan/ExternalProject_libassuan.mk
+++ b/external/libassuan/ExternalProject_libassuan.mk
@@ -31,6 +31,7 @@ $(call gb_ExternalProject_get_state_target,libassuan,build): $(call gb_Executabl
		--disable-shared \
		--disable-doc \
		$(if $(verbose),--disable-silent-rules,--enable-silent-rules) \
		CFLAGS="$(CFLAGS) -D__STDC__=1" \
		CXXFLAGS="$(CXXFLAGS)" \
		GPG_ERROR_CFLAGS="$(GPG_ERROR_CFLAGS)" \
		GPG_ERROR_LIBS="$(GPG_ERROR_LIBS)" \