vlib: add fixes for problems, found by check_os_api_parity.v

vlib/builtin/cfns.c.v

@@ -400,3 +400,6 @@ fn C.pthread_mutex_unlock(voidptr) int
 fn C.read(fd int, buf voidptr, count size_t) int
 fn C.write(fd int, buf voidptr, count size_t) int
 fn C.close(fd int) int
+
+// used by gl, stbi, freetype
+fn C.glTexImage2D()

vlib/gl/gl.v

@@ -53,7 +53,6 @@ fn C.glClear()
 fn C.glCreateShader() int
 fn C.glClearColor()
 fn C.glViewport()
-fn C.glTexImage2D()
 fn C.glPixelStorei()
 fn C.glBlendFunc()
 fn C.glPolygonMode()

vlib/sokol/gfx/gfx.v

@@ -1,5 +1,9 @@
 module gfx
 
+pub const (
+	version = 1
+)
+
 // setup and misc functions
 [inline]
 pub fn setup(desc &C.sg_desc) {

vlib/sokol/sgl/sgl.v

@@ -1,5 +1,10 @@
 module sgl
 
+import sokol.gfx
+
+pub const (
+	version = gfx.version + 1
+)
 
 /* setup/shutdown/misc */
 [inline]