Check for __attribute__((visibility)) support

The solaris compiler does not support the GNU C extension
__attribute__((visibility("hidden"))), leading to build failures.
This commit adds a check for that support and defines NO_VIZ if it's
visibility setting is not supported.

Fixes #852

configure.ac

@@ -209,6 +209,30 @@ if test "$ac_cv_so_sendfile_works" = yes; then
 	AC_DEFINE([SENDFILE_WORKS], [1], [Define if SENDFILE works])
 fi
 
+# Support for visibility attribute 
+save_CFLAGS="${CFLAGS}" 
+CFLAGS="${CFLAGS} -Werror" 
+
+AC_CACHE_CHECK([whether we have support for visibility attributes], 
+	[ac_cv_have_viz], 
+	[AC_RUN_IFELSE( 
+		[AC_LANG_PROGRAM([[ 
+			#if ((__GNUC__-0) * 10 + __GNUC_MINOR__-0 >= 33) 
+			#  define ZLIB_INTERNAL __attribute__((visibility ("hidden"))) 
+			#else 
+			#  define ZLIB_INTERNAL 
+			#endif 
+			int ZLIB_INTERNAL foo; 
+		]],[])], 
+	[ac_cv_have_viz=yes], 
+	[ac_cv_have_viz=no]) 
+])
+if test "$ac_cv_have_viz" = no; then 
+	libvgz_extra_cflags="-DNO_VIZ" 
+	AC_SUBST(libvgz_extra_cflags) 
+fi 
+CFLAGS="${save_CFLAGS}" 
+
 # Userland slab allocator, available only on Solaris
 case $target in
 *-*-solaris*)

lib/libvgz/Makefile.am

@@ -3,7 +3,7 @@
 lib_LTLIBRARIES = libvgz.la
 
 libvgz_la_LDFLAGS = -version-info 1:0:0
-libvgz_la_CFLAGS = -D_LARGEFILE64_SOURCE=1
+libvgz_la_CFLAGS = -D_LARGEFILE64_SOURCE=1 $(libvgz_extra_cflags)
 
 libvgz_la_SOURCES = \
 	adler32.c \