Suffixed source fixes

[mohawk2]

Description

This constitutes an independent part of (and partial preparation for) #1407, fixing these problems:

• the Makefiles currently do not clean all their outputs
• there is no easy self-contained test for GenerateSuffixedSource.cmake

Checklist

• The documentation has been updated.
• If the PR solves a specific issue, it is set to be closed on merge.

[mohawk2]

@ACSimon33 I am hoping this PR will be quite an easy change to accept, and would reduce what I call the "overhang" of PR #1407.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 69.39%. Comparing base (6de9594) to head (24c676c).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1434   +/-   ##
=======================================
  Coverage   69.39%   69.39%           
=======================================
  Files        6122     6122           
  Lines      487623   487623           
  Branches    23268    23268           
=======================================
  Hits       338408   338408           
  Misses     148781   148781           
  Partials      434      434           

Components	Coverage Δ
BLAS	99.13% <ø> (ø)
CBLAS	96.98% <ø> (ø)
LAPACK	82.30% <ø> (ø)
LAPACKE	2.17% <ø> (ø)
TMGLIB	55.69% <ø> (ø)
BLAS testing	88.54% <ø> (ø)
CBLAS testing	89.63% <ø> (ø)
LAPACK testing	82.21% <ø> (ø)
LAPACKE testing	∅ <ø> (∅)

Files with missing lines	Coverage Δ
SRC/cgedmdq.f90	78.19% <ø> (ø)
SRC/dgedmdq.f90	77.34% <ø> (ø)

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