Avoid clash of base64 symbols

On Debian/Ubuntu, one of the libraries libvirt (indirectly) links
with exports a symbol named 'base64_encode'. This takes precedence
over GNULIB's base64_encode function during linking. Unfortunately
they of course have different API semantics. To avoid this problem
use a few #defines in config.h to rename the GNULIB provided
function to have a 'libvirt_gl_' prefix

configure.ac

@@ -2876,6 +2876,11 @@ test "x$lv_cv_static_analysis" = xyes && t=1
 AC_DEFINE_UNQUOTED([STATIC_ANALYSIS], [$t],
   [Define to 1 when performing static analysis.])
 
+# Some GNULIB base64 symbols clash with a kerberos library
+AC_DEFINE_UNQUOTED([isbase64],[libvirt_gl_isbase64],[Hack to avoid symbol clash])
+AC_DEFINE_UNQUOTED([base64_encode],[libvirt_gl_base64_encode],[Hack to avoid symbol clash])
+AC_DEFINE_UNQUOTED([base64_encode_alloc],[libvirt_gl_base64_encode_alloc],[Hack to avoid symbol clash])
+
 AC_OUTPUT(Makefile src/Makefile include/Makefile docs/Makefile \
 	  docs/schemas/Makefile \
 	  gnulib/lib/Makefile \