Log non-fatally if CMake doesn't find a package

If CMake fails to find a package, we should log it in meson-log.txt, but
shouldn't bother the user unless there are details to report.  In
addition, we were calling mlog.warning() without fatal=False, so if msetup
was called with --fatal-meson-warnings we'd fail the entire setup in this
case, even for optional dependencies.  Log a notice with fatal=False if
CMake reported an error, and a debug message otherwise.

Notably, the "even though Meson's preliminary check succeeded" case can
occur when a dependency is missing but its Find*.cmake is shipped with
CMake itself.

Fixes: 92c517ea69 ("output PACKAGE_NOT_FOUND_MESSAGE as warning when CMake package is not found")

mesonbuild/dependencies/cmake.py

@@ -416,11 +416,11 @@ class CMakeDependency(ExternalDependency):
         if not self.is_found:
             not_found_message = self.traceparser.get_cmake_var('PACKAGE_NOT_FOUND_MESSAGE')
             if len(not_found_message) > 0:
-                mlog.warning(
+                mlog.notice(
                     'CMake reported that the package {} was not found with the following reason:\n'
-                    '{}'.format(name, not_found_message[0]))
+                    '{}'.format(name, not_found_message[0]), fatal=False)
             else:
-                mlog.warning(
+                mlog.debug(
                     'CMake reported that the package {} was not found, '
                     'even though Meson\'s preliminary check succeeded.'.format(name))
             raise self._gen_exception('PACKAGE_FOUND is false')