Add note for setting CC and CXX for prebuilt pony installs
Add note for setting CC and CXX for prebuilt pony installs

This adds a note to the ponyc install documentation
to inform users that clang will be used as default
compiler, and gives them instructions on how to
override with their compiler of choice.

Signed-off-by: hasheddan <fff88e83cd964a817f9e74a48f8df9cd0b6b9759@gmail.com>

jemc/ponyc,sgebbie/ponyc,dipinhora/ponyc,sgebbie/ponyc,jemc/ponyc,sgebbie/ponyc,ponylang/ponyc,sgebbie/ponyc,ponylang/ponyc,dipinhora/ponyc,dipinhora/ponyc,jemc/ponyc,sgebbie/ponyc,ponylang/ponyc