math/atlas: unbreak after math/lapack + various fixes

The latest upgrade of math/lapack broke ATLAS, due to the lack of the
post-target, but anyways it was broken in many others ways:

• gmake is required (the fixes ifdef / #ifdef are not sufficient)

• -fPIC is needed everywhere

• fix EXTRA_FFLAGS

• remove a conflict with atlas-devel.

PR: 257122
Reported by: alt2600 (at) icloud.com
MFH: 2021Q3