meson: add patch to fix caching of pkg-config files.

Allows us to use the PKG_CONFIG variable to tell Meson which is the
pkg-config to be used to get our native dependencies.

We actually could always do that but Meson would cache the variable for
the pkg-config file if we used the dependency() call without 'native:
true' and after that all future usages of dependency() even with
'native: true' would use the cross-compile one.

See this issue from where the patch came from:

- https://github.com/mesonbuild/meson/issues/1736

srcpkgs/meson/patches/fix-pkg-config-native-not-found.patch

@@ -0,0 +1,15 @@
+Source: https://github.com/mesonbuild/meson/issues/1736
+Upstream: Unknown
+Reason: prevents meson from caching a variable with the value of the cross-compile pkg-config that can be later used to fetch the value of a native pkg-config, fixes problems with finding native: true dependencies.
+
+--- mesonbuild/dependencies/base.py
++++ mesonbuild/dependencies/base.py
+@@ -448,7 +448,6 @@ class PkgConfigDependency(ExternalDependency):
+                 potential_pkgbin = ExternalProgram.from_cross_info(environment.cross_info, 'pkgconfig')
+                 if potential_pkgbin.found():
+                     self.pkgbin = potential_pkgbin
+-                    PkgConfigDependency.class_pkgbin = self.pkgbin
+                 else:
+                     mlog.debug('Cross pkg-config %s not found.' % potential_pkgbin.name)
+         # Only search for the native pkg-config the first time and
+<Paste>

srcpkgs/meson/template

@@ -1,7 +1,7 @@
 # Template file for 'meson'
 pkgname=meson
 version=0.47.2
-revision=1
+revision=2
 noarch=yes
 build_style=python3-module
 pycompile_module="mesonbuild"