Don't force libstdc++ (fix build w/ libc++)

Rebase of the ghostscript-gpl-patches.git commit a8581432 (Sam James)
onto 10.08.0, where the flag list became a loop that keeps every flag
that compiles, so -stdlib=libstdc++ would still be forced under clang.

Bug: https://bugs.gentoo.org/784173

--- a/configure.ac
+++ b/configure.ac
@@ -932,7 +932,7 @@
         CXXFLAGS_TO_USE=""
 
         AC_MSG_CHECKING([supported C++ compiler flags])
-        for flag in "-std=c++17 -stdlib=libstdc++" "-std=c++17" "-stdlib=libstdc++" ; do
+        for flag in "-std=c++17" ; do
           CXXFLAGS="$CXXFLAGS $flag"
 
           AC_COMPILE_IFELSE([AC_LANG_PROGRAM([