When cross-compiling, rust-bindgen needs the -isysroot
flag we pass to the C++ compiler to find the correct
headers. Add a new BINDGEN_CFLAGS environment variable
for passing this and other relevant options, and reformat
its contents in toolchain.configure so we can use autoconf-
style template substitution to poke it into a bindgen.toml
file to be read by build scripts like build_gecko.rs.
Set this variable from the macosx/cross-mozconfig.common
to the same extra flags we pass to CXX so automation
builds work correctly with --enable-stylo.
MozReview-Commit-ID: 7wabObiFtVb
--HG--
extra : rebase_source : eeba30e3d64112da65e2e6830ef5fc1b54965529