make sure all the compat symbols appear in the shared library.

_GLIBCXX_SHARED is only set for the shared library version so that
in includes compat symbols that are not part of the static library.

noticed when arm64 rust build with gcc14 failed due to a very
strange "can't find symbol" problem (the symbol was present, but
the one with a version attached was not, and that was wanted.)

Author: mrgtwentythree

external/gpl3/gcc/lib/libstdc++-v3/Makefile

@@ -1,4 +1,4 @@
-#	$NetBSD: Makefile,v 1.59 2025/09/19 05:27:45 mrg Exp $
+#	$NetBSD: Makefile,v 1.60 2025/10/14 07:19:46 mrg Exp $
 
 REQUIRETOOLS=	yes
 NOLINT=		# defined
@@ -62,10 +62,9 @@ SRCS=		${LIBSUPCXXSRCS} ${LIBSTDCXXSRCS}
 
 GCC_MACHINE_SUBDIR=${MACHINE_CPU:C/powerpc.*/rs6000/:C/x86_64/i386/}
 
-# The compatibility files are built without -D_GLIBCXX_SHARED.
-.for s in ${SRCS:Ncompatibility.cc:Ncompatibility-debug_list.cc:Ncompatibility-debug_list-2.cc:Ncompatibility-atomic-c++0x.cc:Ncompatibility-c++0x.cc:Ncompatibility-chrono.cc:Ncompatibility-condvar.cc:Ncompatibility-thread-c++0x.cc}
-CSHLIBFLAGS.${s}+=	-D_GLIBCXX_SHARED
-.endfor
+# Only the shared library is build with -D_GLIBCXX_SHARED, and the
+# static library shouldn't need to provide old symbols.
+CSHLIBFLAGS+=	-D_GLIBCXX_SHARED
 
 CXXFLAGS+=	-fno-implicit-templates
 CPPFLAGS+=	-I${DIST}/libstdc++-v3/include