Fix regression that disabled shared library builds

My previous attempt that was supposed to disable shared library builds
with nacl had a side effect which basically disabled shared libs for all
configurations.

Eventhough AC_DISABLE_SHARED was used inside an if clause it seemed to
take over in any case.

I could not find a clean way around this, so had to override internal
libtool variables. Will check with the libtool people regarding a
cleaner implementation.

configure.ac

@@ -62,7 +62,6 @@ AC_ARG_ENABLE([nacl],
             WANT_NACL="no"
         elif test "x$enableval" = "xyes"; then
             WANT_NACL="yes"
-            AC_DISABLE_SHARED
         fi
     ]
 )
@@ -215,6 +214,12 @@ m4_ifdef([AM_PROG_AR], [AM_PROG_AR])
 AC_LIBTOOL_WIN32_DLL
 AC_PROG_LIBTOOL
 
+if test "x$WANT_NACL" = "xyes"; then
+    disable_shared=yes
+    enable_static=yes
+fi
+
+
 WIN32=no
 AC_CANONICAL_HOST
 case $host_os in