gnome module: fix invalid find_tool variable contents causing crash

If a tool that is looked up in a .pc file is supposed to be there and
has a pkg-config variable entry, but the value is incorrect, we can't
actually use it.

Since commit ab3d020 we actually do run
the ExternalProgram search procedure on it though -- which caused it to
go wonky and return a None if it doesn't exist, instead of containing a
path to a program that does not exist and fails at build time. This is
better in the case where searching helps resolve .exe file extensions --
and worse in the case where patches to the dependency means nothing we
do is ever enough to actually find what is expected, since now we crash.

Raise an explicit error in such a case, pointing out that the dependency
itself is broken and needs a distributor-side resolution.

Fixes #12412

mesonbuild/modules/__init__.py

@@ -112,7 +112,12 @@ def find_tool(self, name: str, depname: str, varname: str, required: bool = True
         if dep.found() and dep.type_name == 'pkgconfig':
             value = dep.get_variable(pkgconfig=varname)
             if value:
-                return ExternalProgram(value)
+                progobj = ExternalProgram(value)
+                if not progobj.found():
+                    msg = (f'Dependency {depname!r} tool variable {varname!r} contains erroneous value: {value!r}\n\n'
+                           'This is a distributor issue -- please report it to your {depname} provider.')
+                    raise mesonlib.MesonException(msg)
+                return progobj
 
         # Normal program lookup
         return self.find_program(name, required=required, wanted=wanted)