Explicitly use -std=c99 when compiling libgpg-error

[JamieMagee]

GCC changed the default C standard from gnu17 to gnu23 in GCC 15¹

C23 by default: GCC 15 changes the default language version for C compilation from -std=gnu17 to -std=gnu23. If your code relies on older versions of the C standard, you will need to either add -std= to your build flags, or port your code; see the porting notes.

This started causing errors when compiling libgpg-error

t-printf.c:452:18: error: expected identifier or ‘(’ before ‘nullptr’
  452 |   volatile char *nullptr = NULL; /* Avoid compiler warning.  */
      |                  ^~~~~~~

nullptr is a reserved constant in C23², so the compilation fails.

This change explicitly adds -std=c99 to CFLAGS when compiling libgpg-error to avoid this issue.

This issue was also fixed upstream by renaming nullptr to null_ptr³ and released in libgpg-error 1.51.

Closes #199

Footnotes

1. https://gcc.gnu.org/gcc-15/changes.html#c ↩

2. https://en.cppreference.com/w/c/language/nullptr.html ↩

3. https://github.com/gpg/libgpg-error/commit/d14c69a7f256a9444c5c64b808d8767e5a8ba68e ↩

[ueno]

The bundled libgpg-error is a bit outdated (the latest is 1.55). Perhaps bumping it would make this unnecessary?

[JamieMagee]

Yes, I created #201 which also solves the issue. If you merge that, I'll close this.

[JamieMagee]

Closing as this was resolved in #201