Pass cflags to compile

[ryangraham]

So that I can pass CFLAGS to avoid segfaults in ree-1.8.7-2012.02.

  rbenv::compile { 'ree-1.8.7-2012.02':
    configure_opts => '',
    cflags => '-O2 -fno-tree-dce -fno-optimize-sibling-calls',
    source => 'puppet:///.../patched-ree'
  }

Although I see another pull request for MAKE_OPTS. Maybe rather than adding all of these fields we should make environment more flexible.

PS: Thanks for the great module. :)