Use C locale when building gtktypefuncs.c
We're using [a-z] ranges with sed and grep, and POSIX does not specify their behaviour in non-ASCII locales:
In the POSIX locale, a range expression represents the set of collating elements that fall between two elements in the collation sequence, inclusive. In other locales, a range expression has unspecified behavior -- IEEE Std 1003.1-2017, § 9.3.5 (7)
This can lead to no results, or invalid replacements, which in turn can lead to broken builds or broken build artifacts.