Enable OPTFLAGS to pass custom compile flags to Makefile

[pbleser]

Most distributions use custom compiler flags across all packages.
The following patch introduces the OPTFLAGS Makefile variable, which defaults to upstream's settings (-g -O2) but can be overridden by packagers (e.g. on openSUSE/SLE we use "-fmessage-length=0 -O2 -Wall -D_FORTIFY_SOURCE=2 -fstack-protector -funwind-tables -fasynchronous-unwind-tables -g -Wextra")

https://api.opensuse.org/public/source/X11:Utilities/feh/feh-makefile_optflags.patch?rev=45ef01c1172dd974f39fed346d43073d

Note that it also masks the output of cc for computing dependencies.

[derf]

Unable to reproduce with both gcc 4.6.2 and clang 3.0

Closing since I got no response to my question in #70.