add macports libcxx dependency on older macs for gdb

this will ensure older systems are able to build and use the latest gdb
with minimal effort

hopefully other members on the macports squad will look into the macro
nusiance that otherwise impedes a clean compilation

devel/gdb/Portfile

@@ -86,6 +86,13 @@ pre-configure {
 
 build.dir ${configure.dir}
 
+#older macs require a new libcxx
+if {${os.platform} eq "darwin" && ${os.major} < 15} {
+    configure.depends_lib-append port:macports-libcxx
+    configure.cxxflags-append    -L/opt/local/lib/libcxx
+}
+
+
 post-destroot {
     if {${os.platform} eq "darwin" && ${os.major} < 12} {
         system "chgrp procmod ${destroot}${prefix}/bin/ggdb*"