[buddy] Probe for -fvisibility and -fvisibility-inlines-hidden.

* configure.ac: Check gcc and g++ for -fvisibility and
-fvisibility-inlines-hidden.  Add these options to
CFLAGS and CXXFLAGS.
* m4/ax_check_compile_flag.m4: New file.
* src/Makefile.am: Build BuDDy as a single library, reverting part of
the changes introduced in my previous patch to this file.  Since
the options are set in CFLAGS/CXXFLAGS, there is no possibility
for -fvisibility-inlines-hidden to be passed to the C compiler.

buddy/configure.ac

@@ -20,6 +20,18 @@ AC_PROG_CXX
 
 LT_INIT([win32-dll])
 
+# Use -Werror since using -fvisibility under MinGW is only a warning.
+# (The option is ignored anyway since this does not make sense under windows).
+AX_CHECK_COMPILE_FLAG([-Werror -fvisibility=hidden],
+  [CFLAGS="$CFLAGS -fvisibility=hidden"])
+AC_LANG(C++)
+AX_CHECK_COMPILE_FLAG([-Werror -fvisibility=hidden],
+  [CXXFLAGS="$CXXFLAGS -fvisibility=hidden"
+  AX_CHECK_COMPILE_FLAG([-fvisibility-inlines-hidden],
+    [CXXFLAGS="$CXXFLAGS -fvisibility-inlines-hidden"])])
+AC_LANG(C)
+
+
 adl_ENABLE_DEBUG
 ad_GCC_OPTIM
 adl_NDEBUG
@@ -31,6 +43,7 @@ if test x$enable_warnings = xyes; then
   CF_GCC_WARNINGS
 fi
 
+
 AC_CONFIG_HEADERS([config.h])
 
 AC_CONFIG_FILES([