Skip lz4 check when unnecessary

[freeekanayaka]

This change improves ./configure in a few ways:

• lz4 detection via pkg-config is performed only when it makes sense (i.e. when libuv is available and --without-lz4 is not passed)
• the documentation and checks for --enable-uv / --disable-uv are now more clear about the fact that these flags are about enabling or disabling compression by default, not about enabling or disabling compression support altogether (which is controlled by --with-lz4 / --without-lz4)
• several tests have been put in place to ensure that ./configure succeeds or fails as expected in a number of scenarios

The changes of individual commits should be a bit easier to parse than the full change set.

With these improvements combined, this fixes #441.

[codecov[bot]]

Codecov Report

Merging #442 (0de7a36) into master (ba50497) will decrease coverage by 0.05%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #442      +/-   ##
==========================================
- Coverage   76.72%   76.68%   -0.05%     
==========================================
  Files          51       51              
  Lines        9597     9597              
  Branches     2443     2443              
==========================================
- Hits         7363     7359       -4     
- Misses       1160     1165       +5     
+ Partials     1074     1073       -1     

see 1 file with indirect coverage changes