Swift is now set up to properly respect CFLAGS, CXXFLAGS, and LDFLAGS
from the environment by not overriding those values in the build system.
Swift 6 now also picks up on `SWIFT_EXTRA_BUILD_FLAGS` from the
environment and passes those to its build system as well.
Closes: https://bugs.gentoo.org/939764
Closes: https://bugs.gentoo.org/939766
Signed-off-by: Itai Ferber <itai@itaiferber.net>
