Use -Bsymbolic-functions and -Bsymbolic

This avoids dynamic lookups to resolve symbols inside the library, but
disallows symbol interposition.

* m4/symbolic.m4: New file.
* buddy/m4/symbolic.m4: New link.
* configure.ac, buddy/configure.ac: Add AX_SYMBOLIC.
* buddy/src/Makefile.am, iface/ltsmin/Makefile.am, src/Makefile.am,
wrap/python/Makefile.am: Link with $(SYMBOLIC_LDFLAGS).

m4/symbolic.m4

@@ -0,0 +1,31 @@
+dnl This was addapted from a patch submitted written by H.J. Lu for GCC.
+dnl   https://gcc.gnu.org/ml/gcc/2007-01/msg00363.html
+dnl The -Bsymbolic-functions is no-longer part of the --help of ld, but
+dnl we can call ld -Bsymbolic-functions --help and it will choke if the
+dnl option is not supported.
+dnl Substitute SYMBOLIC_LDFLAGS with -Bsymbolic-functions for GNU linker
+dnl if it is supported.
+AC_DEFUN([AC_LIB_PROG_LD_GNU_SYMBOLIC],
+[AC_CACHE_CHECK([if the GNU linker ($LD) supports -Bsymbolic-functions],
+acl_cv_prog_gnu_ld_symbolic, [
+acl_cv_prog_gnu_ld_symbolic=no
+if test x"$with_gnu_ld" = x"yes"; then
+  if $LD -Bsymbolic-functions -Bsymbolic --help>/dev/null 2>&1 </dev/null; then
+    acl_cv_prog_gnu_ld_symbolic=yes
+  fi
+fi])
+if test x"$acl_cv_prog_gnu_ld_symbolic" = x"yes"; then
+  SYMBOLIC_LDFLAGS="-Wl,-Bsymbolic-functions -Wl,-Bsymbolic"
+else
+  SYMBOLIC_LDFLAGS=''
+fi
+AC_SUBST([SYMBOLIC_LDFLAGS])])
+
+AC_DEFUN([AX_BSYMBOLIC],
+[AC_ARG_ENABLE([Bsymbolic],
+   [AS_HELP_STRING([--disable-Bsymbolic],
+                   [disable linking with -Bsymbolic])],
+   [], [enable_Bsymbolic=yes])
+if test "$enable_Bsymbolic" != "no"; then
+  AC_LIB_PROG_LD_GNU_SYMBOLIC
+fi])