meson: allow dirs outside of prefix

Upstream insists on not allowing bindir and other dir options
outside of prefix for some reason:

mesonbuild/meson#2561

We remove the check so multiple outputs can work sanely.

pkgs/development/tools/build-managers/meson/allow-dirs-outside-of-prefix.patch

@@ -0,0 +1,25 @@
+--- a/mesonbuild/coredata.py
++++ b/mesonbuild/coredata.py
+@@ -266,18 +266,13 @@
+         '''
+         if option.endswith('dir') and os.path.isabs(value) and \
+            option not in builtin_dir_noprefix_options:
+-            # Value must be a subdir of the prefix
+             # commonpath will always return a path in the native format, so we
+             # must use pathlib.PurePath to do the same conversion before
+             # comparing.
+-            if commonpath([value, prefix]) != str(PurePath(prefix)):
+-                m = 'The value of the {!r} option is {!r} which must be a ' \
+-                    'subdir of the prefix {!r}.\nNote that if you pass a ' \
+-                    'relative path, it is assumed to be a subdir of prefix.'
+-                raise MesonException(m.format(option, value, prefix))
+-            # Convert path to be relative to prefix
+-            skip = len(prefix) + 1
+-            value = value[skip:]
++            if commonpath([value, prefix]) == str(PurePath(prefix)):
++                # Convert path to be relative to prefix
++                skip = len(prefix) + 1
++                value = value[skip:]
+         return value
+
+     def init_builtins(self, options):

pkgs/development/tools/build-managers/meson/default.nix

@@ -21,6 +21,12 @@ in python3Packages.buildPythonApplication rec {
   '';
 
   patches = [
+    # Upstream insists on not allowing bindir and other dir options
+    # outside of prefix for some reason:
+    # https://github.com/mesonbuild/meson/issues/2561
+    # We remove the check so multiple outputs can work sanely.
+    ./allow-dirs-outside-of-prefix.patch
+
     # Unlike libtool, vanilla Meson does not pass any information
     # about the path library will be installed to to g-ir-scanner,
     # breaking the GIR when path other than ${!outputLib}/lib is used.