[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/src/Makefile.am

@@ -1,29 +1,21 @@
 
 include_HEADERS = bdd.h fdd.h bvec.h
 
-BUDDY_FLAGS = -I$(top_builddir) -DVERSION=23 \
-  -fvisibility=hidden -DBUDDY_DLL_EXPORTS -DBUDDY_DLL
+BUDDY_FLAGS = -I$(top_builddir) -DVERSION=23 -DBUDDY_DLL_EXPORTS -DBUDDY_DLL
 
-# Compile the C++ part of the library with
-# -fvisibility-inlines-hidden, this option is not valid for C.
 lib_LTLIBRARIES = libbdd.la
-libbdd_la_CPPFLAGS = $(BUDDY_FLAGS) -fvisibility-inlines-hidden
-libbdd_la_SOURCES = cppext.cxx
-libbdd_la_LIBADD = libbdd_c.la
 # See the `Updating version info' node of the Libtool manual before
 # changing this.
 libbdd_la_LDFLAGS = -no-undefined -version-info 0:0:0
-
-# The C part of the library.
-noinst_LTLIBRARIES = libbdd_c.la
-libbdd_c_la_CPPFLAGS = $(BUDDY_FLAGS)
-libbdd_c_la_SOURCES = \
+libbdd_la_CPPFLAGS = $(BUDDY_FLAGS)
+libbdd_la_SOURCES = \
  bddio.c \
  bddop.c \
  bddtree.h \
  bvec.c \
  cache.c \
  cache.h \
+ cppext.cxx \
  fdd.c \
  imatrix.c \
  imatrix.h \