[Fix] nvm should respect CC/CXX env var
[Fix] nvm should respect CC/CXX env var

 - Also set CC for Freebsd, not only CXX

 - Also set CC=cc, CXX=c++ on Mac OS by default as upstream

Reference:
https://github.com/nodejs/node/blob/1bd871655a8b76fa3be1e3c6c325efa74c86fcd9/configure#L16-L17

edwmurph/nvm,evocateur/nvm,ljharb/nvm,PeterDaveHelloKitchen/nvm,ljharb/nvm,PeterDaveHelloKitchen/nvm,PeterDaveHello/nvm,creationix/nvm,PeterDaveHello/nvm,evocateur/nvm,edwmurph/nvm,creationix/nvm