fix global visibility (vis.h) support for out-of-tree builds

commit 2f853dd6b9a95d5b13ee8f9df762125e0588df5d failed to change the
test for -include vis.h support to use $srcdir, so vis.h was always
disabled by configure for out-of-tree builds.

configure

@@ -517,7 +517,7 @@ int foo(void) { }
 int bar(void) { fp = foo; return foo(); }
 EOF
 if $CC $CFLAGS_C99FSE $CPPFLAGS $CFLAGS \
-  -DSHARED -fPIC -I./src/internal -include vis.h \
+  -DSHARED -fPIC -I$srcdir/src/internal -include vis.h \
   -nostdlib -shared -Wl,-Bsymbolic-functions \
   -o /dev/null "$tmpc" >/dev/null 2>&1 ; then
 visibility=yes