[Chapel Merge] Fix how we stub out GNU's __extension__ in c2chape

Branch: refs/heads/main
Revision: 6168898
Author: bradcray
Log Message:

Merge pull request #18058 from bradcray/fix-c2chapel-gnu-extension

Fix how we stub out GNU's extension in c2chapel

[reviewed by @ronawho]

This reflects how the pycparser FAQ says to stub out GNU's __extension__
and seems to work better when trying to parse glib header files like those
used by Apache Arrow.

[Note that Elliot points out that our implementation matches what pycparser's FAQ
said at that time]

Modified Files:
M tools/c2chapel/utils/custom.h

Compare: https://github.com/chapel-lang/chapel/compare/204df6d8cda2...6168898189fd