Add option to force/disable `-Werror` for a build

[pwithnall]

Meson has a --werror option, and autotools has a --disable-Werror option (if using AX_COMPILER_FLAGS). For automated build systems (nightly builds, etc.) it would be good to add an option to flatpak-builder which was passed through to those two flags, to either enable -Werror, or force it to be disabled.

Situations where you might want to force-enable -Werror include when a developer is building locally. Situations where you might want to force-disable it include nightly builds and CI systems which don’t want to check for regressions on warnings.

[TingPing]

and autotools has a --disable-Werror option (if using AX_COMPILER_FLAGS)

Well flatpak-builder can't know if thats used or not and would just be passing potentially problem causing arguments (if it is a custom configure script for example).

EDIT: Well it could parse the script to see if it uses it but it doesn't currently.

[pwithnall]

Could add something like config-opts which specifies the config options to pass to disable/enable -Werror. Or parse the configure.ac, as you say.