gcc7/libgcc7: clear MacPorts optflags

optflag Os causes gcc7 build to fail bootstrapping with older gcc compilers
use gcc default optflags

we probably will need to do the same to other gcc versions 7+
we probably also want to clear the other potentially-troublesome configure.XYZflags

fixes: https://trac.macports.org/tickets/59706

lang/gcc7/Portfile

@@ -359,6 +359,12 @@ configure.env-append \
 build.env-append \
     "CPP=${configure.cc} -E" \
     "CXXCPP=${configure.cxx} -E"
+
+# gcc default optflags are required to build gcc7 with older compilers
+# see https://trac.macports.org/ticket/599706
+# we probably also want to clear out the rest of the configure.XYZ flags
+configure.optflags
+
 configure.cc-append [get_canonical_archflags]
 configure.cc_archflags
 configure.cxx-append ${configure.cxx_archflags}