Adds option to set the compiler for source builds.

gcc 4.7 and later change the default compile flags, resulting in warnings which stop the build process.
This change allows the compiler to be set (as per Phusion documents), along with the flags. For Ubuntu 13.10 this should be:
"nginx": {
  "source": {
     "compiler_cc": "/usr/bin/gcc -Wno-unused-local-typedefs -Wno-type-limits",
     "compiler_cxx": "/usr/bin/g++ -Wno-unused-local-typedefs -Wno-type-limits"
  }
}