databases/grass7: fix build on GCC architectures

Force C11 compiler:
checking whether the C compiler (cc -O2 -pipe -std=c11 -DLIBICONV_PLUG -fstack-protector-strong -fno-strict-aliasing -L/usr/local/lib -L/usr/local/lib -L/usr/local/lib -fstack-protector-strong ) works... no