gnu: meson: Patch to allow installing to independent prefixes.

Meson goes to lengths at preventing installation to directories outside of the
main installation prefix.  This isn't convenient or desirable in Guix; patch
it out using a patch maintained by Nix.

* gnu/packages/patches/meson-allow-dirs-outside-of-prefix.patch: Add patch.
* gnu/local.mk (dist_patch_DATA): Register it.
* gnu/packages/build-tools.scm (meson): Use it.

gnu/local.mk

@@ -1468,6 +1468,7 @@ dist_patch_DATA =						\
   %D%/packages/patches/mesa-opencl-all-targets.patch		\
   %D%/packages/patches/mesa-skip-tests.patch			\
   %D%/packages/patches/mescc-tools-boot.patch			\
+  %D%/packages/patches/meson-allow-dirs-outside-of-prefix.patch	\
   %D%/packages/patches/metabat-fix-compilation.patch		\
   %D%/packages/patches/mhash-keygen-test-segfault.patch		\
   %D%/packages/patches/minetest-add-MINETEST_MOD_PATH.patch	\

gnu/packages/build-tools.scm

@@ -272,7 +272,9 @@ files and generates build instructions for the Ninja build system.")
                                   version ".tar.gz"))
               (sha256
                (base32
-                "0v9m0bazhj48kwc8x3gkxg5c3kcvknvqfjlq22z6pm2h2r2nln6v"))))
+                "0v9m0bazhj48kwc8x3gkxg5c3kcvknvqfjlq22z6pm2h2r2nln6v"))
+              (patches (search-patches
+                        "meson-allow-dirs-outside-of-prefix.patch"))))
     (build-system python-build-system)
     (arguments
      `(;; FIXME: Tests require many additional inputs and patching many

gnu/packages/patches/meson-allow-dirs-outside-of-prefix.patch

@@ -0,0 +1,20 @@
+Source: https://raw.githubusercontent.com/NixOS/nixpkgs/master/pkgs/development/tools/build-managers/meson/allow-dirs-outside-of-prefix.patch
+--- a/mesonbuild/coredata.py
++++ b/mesonbuild/coredata.py
+@@ -506,7 +506,6 @@ class CoreData:
+             return value
+         if option.name.endswith('dir') and value.is_absolute() and \
+            option not in BULITIN_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.
+@@ -518,7 +517,7 @@ class CoreData:
+             try:
+                 value = value.relative_to(prefix)
+             except ValueError:
+-                raise MesonException(msg.format(option, value, prefix))
++                pass
+             if '..' in str(value):
+                 raise MesonException(msg.format(option, value, prefix))
+         return value.as_posix()