package/pkg-cmake.mk: force check_language(CXX) to be false when building w/o C+++

See the code snippet below, which typically is used to check if
C++ support can be enabled.

If we manually set CMAKE_CXX_COMPILER to /bin/false, then cmake
will assume that it's fine, without having a real check. Otherwise,
it will do a test run but somehow it falls back to /bin/c++, even
when cross-compiling. Fix that by setting CXX to /bin/false.

```cmake
include(CheckLanguage)
check_language(CXX)

if(CMAKE_CXX_COMPILER)
    enable_language(CXX)
endif()
```

Signed-off-by: Thomas Devoogdt <thomas@devoogdt.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/pkg-cmake.mk

@@ -102,6 +102,7 @@ define $(2)_CONFIGURE_CMDS
 	cd $$($$(PKG)_BUILDDIR) && \
 	rm -f CMakeCache.txt && \
 	PATH=$$(BR_PATH) \
+	$$(if $$(BR2_INSTALL_LIBSTDCPP),,CXX=/bin/false) \
 	$$($$(PKG)_CONF_ENV) $$(BR2_CMAKE) $$($$(PKG)_SRCDIR) \
 		-G$$($$(PKG)_GENERATOR) \
 		-DCMAKE_MAKE_PROGRAM="$$($$(PKG)_GENERATOR_PROGRAM)" \