backends: Avoid whitespace in passed --target-glib

To support e.g. '>=x' vs. '>= x' without passing a strange-looking value
in the latter case. The Vala compiler handles it, but it easily causes
confusion when debugging build system issues.

mesonbuild/backend/backends.py

@@ -1050,7 +1050,7 @@ def generate_basic_compiler_args(self, target: build.BuildTarget, compiler: 'Com
                     if dep.name == 'glib-2.0' and dep.version_reqs is not None:
                         for req in dep.version_reqs:
                             if req.startswith(('>=', '==')):
-                                commands += ['--target-glib', req[2:]]
+                                commands += ['--target-glib', req[2:].lstrip()]
                                 break
                     commands += ['--pkg', dep.name]
                 elif isinstance(dep, dependencies.ExternalLibrary):