Log Message:
sysutils/coreutils: Fix compiler check via pkglint

AUTOFIX: Makefile.common:51: Replacing "${PKGSRC_COMPILER} == \ 
\"clang\"" with "${PKGSRC_COMPILER:Mclang}".

The PKGSRC_COMPILER can be a list of chained compilers, e.g. "ccache
distcc clang". Therefore, comparing it using == or != leads to wrong
results in these cases.