Use an ENV variable for mrbc to help cross-compiling mruby

mruby build system uses mrbc apparently during the build process.
Cross-compiling mruby for ARM using the Android NDK fails because
it tries to use the mrbc ARM binary in x86.

Build output snippet:

    build/host/bin/mrbc: 1: build/host/bin/mrbc: Syntax error: word
    unexpected (expecting ")")
    ...
    ...
    /home/rubiojr/android/toolchain-p14/bin/../lib/gcc/arm-linux-androideabi/4.6/../../../../arm-linux-androideabi/bin/ld:
    build/host/lib/libmruby.a(mrblib.o): in function
    mrb_init_mrblib:mrblib.c(.text+0x40): error: undefined reference to
    'mrblib_irep'

Not sure if adding an ENV var is the right approach though.

I've detailed the whole build process (with the patch) at:

http://rubiojr.rbel.co/hack/2013/01/15/cross-compiling-mruby-for-android-in-debianubuntu-amd64-using-the-android-ndk/