add O3 and -g flags for ruby

Setting any CFLAGS completely disables ruby's default CFLAG settings. On
my mac I have flags like:

    " -O3 -ggdb -Wextra -Wno-unused-parameter -Wno-parentheses
    -Wno-long-long -Wno-missing-field-initializers -Werror=pointer-arith
    -Werror=write-strings -Werror=declaration-after-statement
    -Werror=shorten-64-to-32 -Werror=implicit-function-declaration  -pipe"

But Omnibus currently builds ruby with no optimization and no gdb
support:

    "-I/opt/chef/embedded/include -fPIC"

This patch restores the default flags that ruby normally uses when we're compiling with gcc. Sun studio is left alone.