configure: check for libstdc++.{so,dylib,a}

It's probably a good thing to check for at least one instance of
libstdc++.{so,dylib,a} at configure time. At least one is needed
by the companion libraries and gcc.

Original patch by Bryan Hundven <bryanhundven@gmail.com>

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>

configure

@@ -430,6 +430,13 @@ ncurses_libs="$( for l in ncursesw ncurses curses; do   \
 has_or_abort lib="${ncurses_libs}"                  \
              err="'ncurses' library was not found"
 
+stdcxx_libs="$( for x in so dylib a; do \
+                   printf "libstdc++.$x "; \
+               done \
+             )"
+has_or_abort lib="${stdcxx_libs}" \
+             err="'libstdc++' shared library was not found"
+
 #---------------------------------------------------------------------
 # Compute the version string