top-level configure.ac: factor the libgomp check for posix-like OS

[Gary Funck]

Ended up using the same approach as libatomic,
moving the checking logic into libgupc/configure.tgt.

+# Disable libgupc on unsupported systems.
+if test -d ${srcdir}/libgupc; then
+    if test x$enable_libgupc = x; then
+	AC_MSG_CHECKING([for libgupc support])
+	if (srcdir=${srcdir}/libgupc; \
+		. ${srcdir}/configure.tgt; \
+		test -n "$UNSUPPORTED")
+	then
+	    AC_MSG_RESULT([no])
+	    noconfigdirs="$noconfigdirs target-libgupc"
+	else
+	    AC_MSG_RESULT([yes])
+	fi
+    fi
+fi
+

Thanks,
- Gary