Avoid building unnecessary demos and tests.

The pixmap package does not install neither the demos nor the tests it
includes, yet these get built as part of "make all". Disable them to
shorten build times and, particularly, to workaround ld(1) crashes
triggered by them in powerpc64; see PR bin/202284 for details.

Reviewed by: kwm (maintainer), bdrewery (mentor)
Differential Revision: https://reviews.freebsd.org/D3372