Build improvments part 1

[randombit]

For #1237, first the simple ones:

Add --without-documentation flag to disable building/installing any docs

Split makefile variables so CXXFLAGS can be overridden by downstream without affecting flags we absolutely require.

[codecov-io]

Codecov Report

Merging #1318 into master will decrease coverage by 0.36%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1318      +/-   ##
==========================================
- Coverage   92.14%   91.77%   -0.37%     
==========================================
  Files         766      766              
  Lines       46503    46503              
  Branches     5382     5382              
==========================================
- Hits        42849    42677     -172     
- Misses       3621     3793     +172     
  Partials       33       33

Impacted Files	Coverage Δ
.../block/threefish/threefish_avx2/threefish_avx2.cpp	0% <0%> (-100%)	⬇️
src/lib/rng/rdrand_rng/rdrand_rng.cpp	10% <0%> (-83.34%)	⬇️
src/lib/entropy/rdrand/rdrand.cpp	42.85% <0%> (-57.15%)	⬇️
src/lib/rng/rdrand_rng/rdrand_rng.h	0% <0%> (-28.58%)	⬇️
src/lib/tls/msg_session_ticket.cpp	78.94% <0%> (-21.06%)	⬇️
src/lib/utils/cpuid/cpuid_x86.cpp	76.92% <0%> (-5.77%)	⬇️
src/tests/test_rng.cpp	89.06% <0%> (-3.44%)	⬇️
src/lib/tls/tls_client.cpp	83.4% <0%> (-1.71%)	⬇️
src/lib/block/threefish/threefish.cpp	90.81% <0%> (-1.03%)	⬇️
src/lib/tls/tls_server.cpp	83.43% <0%> (-0.94%)	⬇️
... and 8 more

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