Linux: Default to builtin openssl, freetype, and zlib

Fix support for cross-compilation, and fix handling of builtin libraries,
which would still attempt to link system libs and use system headers.

Also patch out GLU includes from GLEW, we don't need it, so that removes
another mandatory dependency.

drivers/gl_context/glew-remove-glu-dependency.patch

@@ -0,0 +1,22 @@
+diff --git a/drivers/gl_context/glew.h b/drivers/gl_context/glew.h
+index 4eed47708f..5f937862bf 100644
+--- a/drivers/gl_context/glew.h
++++ b/drivers/gl_context/glew.h
+@@ -1168,12 +1168,17 @@ GLAPI void GLAPIENTRY glViewport (GLint x, GLint y, GLsizei width, GLsizei heigh
+ 
+ /* ---------------------------------- GLU ---------------------------------- */
+ 
++// -- GODOT start --
++// We don't use GLU, so remove a dependency.
++#if 0
+ /* this is where we can safely include GLU */
+ #if defined(__APPLE__) && defined(__MACH__)
+ #include <OpenGL/glu.h>
+ #else
+ #include <GL/glu.h>
+ #endif
++#endif
++// -- GODOT end --
+ 
+ /* ----------------------------- GL_VERSION_1_2 ---------------------------- */
+