Makefile: Include a .make.options if present

A file .make.options will be loaded if present. It allows specifyong host
dependent flags without polluting the makefile, or use flags that wont work
on all hosts.
My current one contains:

export CC = clang
export CFLAGS = -march=corei7-avx

Signed-off-by: Michel Pollet <buserror@gmail.com>