build: Ensure that linker requested in link_language is available

Meson was running into an key lookup failure when evaluating the
following:
    project('proj', 'c')
    executable('bin', link_language: 'cpp')
and since 41eb18d also if the executable contained exclusively header
sources.

Author: sp1ritCS

mesonbuild/build.py

@@ -882,6 +882,11 @@ def process_compilers_late(self) -> None:
         # did user override clink_langs for this target?
         link_langs = [self.link_language] if self.link_language else clink_langs
 
+        if self.link_language:
+            if self.link_language not in self.all_compilers:
+                m = f'Target {self.name} requires {self.link_language} compiler not part of the project'
+                raise MesonException(m)
+
         # If this library is linked against another library we need to consider
         # the languages of those libraries as well.
         if self.link_targets or self.link_whole_targets:

test cases/failing/135 rust link_language/meson.build

@@ -1,7 +1,7 @@
 # SPDX-license-identifer: Apache-2.0
 # Copyright © 2021 Intel Corporation
 
-project('rust wrong link language')
+project('rust wrong link language', 'c')
 
 if not add_languages('rust', required: false)
   error('MESON_SKIP_TEST test requires rust compiler')