package/proj: fix build failure due to gcc bug 68485(C++)

package/proj: fix build failure due to gcc bug 68485(C++)

The proj package exhibits gcc bug 68485 when building for the
Microblaze architecture C files (which was already worked around), but
also for C++ files.

As done for other packages in Buildroot work around this gcc bug by
setting optimization to -O0 when compiling C++ files too if
BR2_TOOLCHAIN_HAS_GCC_BUG_68485=y.

Fixes:
http://autobuild.buildroot.net/results/dc9/dc9bc52ff3d5a83dcfe4a86a391590bef57e1cf0/

Signed-off-by: Giulio Benetti <giulio.benetti@benettiengineering.com>
[Thomas: rework implementation to use a single condition for both CFLAGS and CXXFLAGS]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>