external/epoxy: Work around missing clang-cl DLL-init CONSTRUCT support

Looks like neither the __GNUC__ specific __attribute__((constructor)) definition
of CONSTRUCT nor the _MSC_VER specific one (if the #ifs were reordered so that
clang-cl would pick the latter) with __declspec(allocate(".CRT$XCU")) are
supported by clang-cl, and both are rather silently ignored.  That means that
library_init is not called, library_initialized remains false, and the first
call to get_dlopen_handle aborts.

But this whole "verify that get_dlopen_handle isn't called too early" business
is somewhat pointless here (and that's the only use of the CONSTRUCT macro, and
DESTRUCT isn't used at all), so just short-circuit it for clang-cl for now.

Change-Id: I5d50df3574d350f9591e807ef0fb6a1b02dc34ec

external/epoxy/UnpackedTarball_epoxy.mk

@@ -16,6 +16,7 @@ $(eval $(call gb_UnpackedTarball_set_patchlevel,epoxy,0))
 $(eval $(call gb_UnpackedTarball_add_patches,epoxy, \
     external/epoxy/epoxy.windows.api.patch \
     external/epoxy/epoxy.noegl.by.default.patch \
+    external/epoxy/clang-cl.patch \
 ))
 
 # vim: set noet sw=4 ts=4:

external/epoxy/clang-cl.patch

@@ -0,0 +1,14 @@
+--- src/dispatch_common.c
++++ src/dispatch_common.c
+@@ -190,7 +190,11 @@
+ #endif
+ };
+ 
++#if defined _MSC_VER && defined __clang__
++static bool library_initialized = true;
++#else
+ static bool library_initialized;
++#endif
+ 
+ static bool epoxy_current_context_is_glx(void);
+