autotools: relax dist checking

When trying to add provideruoa.so compiled on Ubuntu Quantal to
the archive compiled on Ubuntu Lucid, undefined references to
future libstdc++ tripped up the symbol check.

This is actually okay for system libraries, the .so will simply
not load and/or work on older systems. Filter out such warnings,
starting with libstdc++. Other system libs can be added as needed.

Makefile.am

@@ -361,6 +361,10 @@ if NEON_COMPATIBILITY
 # libneon is intentionally not linked against, to choose between
 # GNUTLS and OpenSSL at runtime.
 LINK_CHECK_ALLOWED += -e 'symbol ne_.*syncdav.so'
+# Allow undefined references to libstdcxx. This happens when
+# adding backends compiled on more recent Linux distros into
+# the release archive.
+LINK_CHECK_ALLOWED += -e '@GLIBCXX_[^ ]* used by'
 endif
 
 # Be strict about running 'syncevolution' only when not doing