Filter certain compile-time options into -DATF_BUILD_*

Items filtered through are:

• Constant defines (-D)
• Include flags (-I)
• Linker flags (-L)
• Optimization level (-O)
• Warnings / linker flags (-W)
• Preprocessor options (-f)

This fixes the scenario hit by the Jenkins job where it's infecting
the build with --sysroot, etc options from the Jenkins build in the
tests.

Prefix all intermediate variables (_CFLAGS, etc) with "ATF_BUILD" [*].

MFC after: 2 weeks
Reported by: Jenkins
Requested by: jmmv
Reviewed by: jmmv
Sponsored by: EMC / Isilon Storage Division
Differential Revision: https://reviews.freebsd.org/D7702