net/p5-Socket6: Fix compiler check via pkglint

AUTOFIX: Makefile:24: Replacing "${PKGSRC_COMPILER} == \"xlc\"" with "${PKGSRC_COMPILER:Mxlc}".

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.

(gdt)