Makefile install need fix

[ulfhanebutte]

to properly create the lib and include directory and copy the files over, the Makefile needs to be fixed
PREFIX needs to be in brackets and directory need to be made if non existing.

here is the fix:
install: all
@mkdir -p $(PREFIX)
@mkdir -p $(PREFIX)/lib
@mkdir -p $(PREFIX)/include
install src/libpcg_random.a $(PREFIX)/lib/.
install -m 0644 include/pcg_variants.h $(PREFIX)/include/.

[imneme]

I agree about the parens, but I don't think it's unreasonable to assume that the library directory should exist. I'm reluctant to run mkdir for the library directory because it's not clear what the right permissions for it would be.