Change c.std value to standard c99

[wroyca]

Closes #10

Looking at the source, they use proper #define to manage with and/or without GNU extension

CI

[helmesjo]

@boris-kolpackov Is this good to go? Looked quickly and there are a few warnings that probably should be taken care of, but they seem unrelated to this PR anyways:

warning: multiple files are distributed as redist/glfw-3.3.8/src/context.c
  info: second file is dist/glfw-3.3.8/src/context.c
  info: this warning will become error in the future

[wroyca]

@boris-kolpackov Is this good to go? Looked quickly and there are a few warnings that probably should be taken care of, but they seem unrelated to this PR anyways:

warning: multiple files are distributed as redist/glfw-3.3.8/src/context.c
  info: second file is dist/glfw-3.3.8/src/context.c
  info: this warning will become error in the future

but they seem unrelated to this PR anyways:

They are indeed unrelated, so I opted to leave them as-is for this PR 👍

[Swat-SomeBug]

Seems good to me

[boris-kolpackov]

Looks good to me. Someone just needs to merge this and release and publish a revision.

[boris-kolpackov]

@Swat-SomeBug Thanks for merging and releasing the 3.3.8+1 revision! It is now published to the testing section of cppget.org: https://cppget.org/glfw/