Fix --enable-shared-glapi configure option

Fix a typo which meant that --enable-shared-glapi didn't actually cause a shared glapi to be built

Signed-off-by: Jon TURNEY <jon.turney@dronecode.org.uk>
diff --git a/configure.ac b/configure.ac
index 46d3516..e1702ba 100644
--- a/configure.ac
+++ b/configure.ac
@@ -614,7 +614,7 @@
 GALLIUM_STATE_TRACKERS_DIRS=""
 
 # build shared-glapi if enabled for OpenGL or if OpenGL ES is enabled
-case "x$enabled_shared_glapi$enable_gles1$enable_gles2" in
+case "x$enable_shared_glapi$enable_gles1$enable_gles2" in
 x*yes*)
     CORE_DIRS="$CORE_DIRS mapi/shared-glapi"
     ;;