Fix the build when liblz4 is missing and test it

[cole-miller]

The build was broken when --enable-build-raft was passed and liblz4 was missing due to a logic error in configure.ac. Fix this and test this particular build configuration in CI.

I only noticed this because we don't install liblz4-dev for the PPA builds and it's preventing setting --enable-build-raft there (canonical/dqlite-ppa#7).

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 77.88%. Comparing base (27eeeaf) to head (1dc45ba).
Report is 71 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #693      +/-   ##
==========================================
- Coverage   77.97%   77.88%   -0.10%     
==========================================
  Files         197      197              
  Lines       28066    28007      -59     
  Branches     5544     5538       -6     
==========================================
- Hits        21885    21812      -73     
- Misses       4329     4349      +20     
+ Partials     1852     1846       -6     

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