Closes gh-18167

[skip ci]

Building BLAS and LAPACK from sources in a NumPy build makes
very little sense in 2021. Therefore remove the documentation
for this from `site.cfg.example`, and doc-deprecate it in
`distutils/system_info.py`.

It's hard to properly deprecate and not worth doing, just
de-emphasizing is enough.

[ci skip]

Generalize the ILP64 BLAS/LAPACK symbol name handling to deal with
arbitrary prefix/suffix.

The build-time behavior is changed so that HAVE_BLAS_ILP64 and
BLAS_SYMBOL_SUFFIX/PREFIX defines are added to compile options
as appropriate.

Mainly to make autodetection of BLAS/LAPACK easier for downstream
numpy.distutils users, add get_info aliases 'blas_ilp64_opt',
'blas_ilp64_plain_opt', and 'blas64__opt' for any/no/""&"64_"
prefix&suffix, and the same for lapack. (Due to the way system_info
works, each also gets a separate class.)

In addition to openblas64_ which has a fixed suffix, add the (by default
suffixless) openblas_ilp64, which correspond to the most likely cases to
be present.

This is an emerging "standard" for 64-bit BLAS/LAPACK, avoiding symbol
clashes with 32-bit BLAS/LAPACK, originally introduced for Julia.
OpenBLAS can be compiled with 64-bit integer size and symbol suffix
'64_' (INTERFACE64=1 SYMBOLSUFFIX=64_).  OpenBLAS built with this
setting is also provided by some Linux distributions (e.g. Fedora's
64-bit openblas packages).