do not repeat magic regexes for cmake define replacements

We already have this magic string in mesonlib, and this should always
have used the cmake@ format which is identical to the meson format other
than the regex.

mesonbuild/modules/cmake.py

@@ -310,10 +310,10 @@ class CmakeModule(ExtensionModule):
             raise mesonlib.MesonException(f'Could not read input file {infile}: {e!s}')
 
         result = []
-        regex = re.compile(r'(?:\\\\)+(?=\\?@)|\\@|@([-a-zA-Z0-9_]+)@')
+        regex = mesonlib.get_variable_regex('cmake@')
         for line in data:
             line = line.replace('@PACKAGE_INIT@', package_init)
-            line, _missing = mesonlib.do_replacement(regex, line, 'meson', confdata)
+            line, _missing = mesonlib.do_replacement(regex, line, 'cmake@', confdata)
 
             result.append(line)