devel/poco: Fix compiler check via pkglint

AUTOFIX: Makefile.common:49: Replacing "${PKGSRC_COMPILER} == \"sunpro\"" with "${PKGSRC_COMPILER:Msunpro}".

        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)