CMake builds: set a default build type (Release)

[tautschnig]

In absence of a default build type, CMake would not use any pre-defined
build configuration. As a consequence, any user running CMake without
setting -DCMAKE_BUILD_TYPE would end up with builds without
optimisation, and thus (surprisingly!) poor performance.

• Each commit message has a non-empty body, explaining why the change was made.
• n/a Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• n/a The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
• n/a My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• n/a White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[codecov]

Codecov Report

Merging #5646 (2a17a11) into develop (4cc9611) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop    #5646   +/-   ##
========================================
  Coverage    69.54%   69.54%           
========================================
  Files         1243     1243           
  Lines       100701   100701           
========================================
  Hits         70037    70037           
  Misses       30664    30664           

Flag	Coverage Δ
cproversmt2	43.32% <ø> (ø)
regression	66.44% <ø> (ø)
unit	32.25% <ø> (ø)

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[martin-cs]

This seems like a reasonable change for a reasonable reason. I don't know CMake well enough to know if this is the best way of doing it. Can we simplify the build instructions using CMake?

[hannes-steffenhagen-diffblue]

This is harmless, but normally a user should specify a build type and if our documentation doesn't say so we should fix that.

[tautschnig]

This is harmless, but normally a user should specify a build type and if our documentation doesn't say so we should fix that.

We didn't clearly state this in our documentation. I have now tried to fix that as well.