package/pkg-meson.mk: fix setting host C++ compiler

package/pkg-meson.mk: fix setting host C++ compiler

Commit f4a61d1ae23ec7729af3a8a165bbee45b6b9ef75 introduced CC_FOR_BUILD and
CXX_FOR_BUILD to avoid detecting ccache.

Both values are set to `HOSTCC`. This causes issues where C++ files are
compiled with the C compiler without passing the `stdc++` flag to the
linker, too.

Therefore, switch to pass the C++ compiler to CXX_FOR_BUILD.

Correctly fixes:
http://autobuild.buildroot.org/results/871e1362c44e5b68a149e6a5dd3caf99ea0d904a

Commit 9783c04aaf5e4fc94099772f1dc699a974ee6538 proposed a fix which in
fact is a workaround to get Meson to pass the `stdc++` flag to the C
linker.

A fellow-up commit will revert this commit, as it is no longer
needed.

Signed-off-by: Jörg Krause <joerg.krause@embedded.rocks>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>