source-highlight: fix build with gcc

only add -stdlib if compiler is clang
closes: https://trac.macports.org/ticket/56089

textproc/source-highlight/Portfile

@@ -28,7 +28,9 @@ checksums       rmd160  a6e0ad1713deb2056984a97adc7dfd0e6610809f \
 # Make sure that the port does not link against system libstdc++.
 # libtool strips unknown flags in link mode, c.f.,
 # https://www.gnu.org/software/libtool/manual/html_node/Stripped-link-flags.html
-configure.ldflags-append    -Wc,-stdlib=${configure.cxx_stdlib}
+if {[string match *clang* ${configure.compiler}]} {
+    configure.ldflags-append    -Wc,-stdlib=${configure.cxx_stdlib}
+}
 
 configure.args  --infodir=${prefix}/share/info \
                 --mandir=${prefix}/share/man \