tools: meson: don't use "allow-shlib-undefined" with LLVM if not supported

From https://github.com/mesonbuild/meson/pull/5912

This solves the -lpthread detection issue with libplacebo when compiling with
LLVM for Windows.

extras/package/win32/build.sh

@@ -111,7 +111,7 @@ mkdir -p extras/tools
 cd extras/tools
 
 export PATH="$PWD/build/bin":"$PATH"
-# Force meson 0.51.1 as newer versions don't add -lpthread properly in libplacebo.pc
+# Force patched meson as newer versions don't add -lpthread properly in libplacebo.pc
 FORCED_TOOLS="meson"
 # Force libtool build when compiling with clang
 if [ "$COMPILING_WITH_CLANG" -gt 0 ] && [ ! -d "libtool" ]; then

extras/tools/meson-shlib-undefined.patch

@@ -0,0 +1,21 @@
+--- meson/mesonbuild/linkers.py.shlib	2020-01-31 11:54:36.928015800 +0100
++++ meson/mesonbuild/linkers.py	2020-01-31 11:54:55.088659900 +0100
+@@ -659,8 +659,17 @@ class LLVMDynamicLinker(GnuLikeDynamicLi
+     This is only the posix-like linker.
+     """
+ 
+-    pass
++    def __init__(self, *args, **kwargs):
++        super().__init__(*args, **kwargs)
+ 
++        # Some targets don't seem to support this argument (windows, wasm, ...)
++        _, _, e = mesonlib.Popen_safe(self.exelist + self._apply_prefix('--allow-shlib-undefined'))
++        self.has_allow_shlib_undefined = not ('unknown argument: --allow-shlib-undefined' in e)
++
++    def get_allow_undefined_args(self) -> T.List[str]:
++        if self.has_allow_shlib_undefined:
++            return self._apply_prefix('--allow-shlib-undefined')
++        return []
+ 
+ class CcrxDynamicLinker(DynamicLinker):
+ 

extras/tools/tools.mak

@@ -410,6 +410,7 @@ meson-$(MESON_VERSION).tar.gz:
 
 meson: meson-$(MESON_VERSION).tar.gz
 	$(UNPACK)
+	$(APPLY) $(TOOLS)/meson-shlib-undefined.patch
 	$(MOVE)
 
 .buildmeson: meson