cmake: use GNUInstallDirs to normalize libexecdir

[trws]

fixes: #1083
Problem: install using wrong libexec directory

Solution: use the libexec directory provided by GNUInstallDirs, add flux intermediary

[codecov]

Codecov Report

Merging #1088 (c3248d2) into master (4e3afdd) will decrease coverage by 0.1%.
The diff coverage is n/a.

❗ Current head c3248d2 differs from pull request most recent head 7d34b0b. Consider uploading reports for the commit 7d34b0b to get more accurate results

@@           Coverage Diff            @@
##           master   #1088     +/-   ##
========================================
- Coverage    71.7%   71.7%   -0.1%     
========================================
  Files          89      89             
  Lines       11665   11665             
========================================
- Hits         8374    8371      -3     
- Misses       3291    3294      +3     

see 1 file with indirect coverage changes

[grondo]

I think this was superseded by a fix in the PR that resurrected make deb (#1082). Closing this one (feel free to reopen if necessary). Thanks for all your help on the build system @trws!