Fix cmake flag in COMPILING.md and other docs

[karkhaz]

The -H flag to cmake was never officially supported; the -S flag should be used instead to specify the source directory. Using -H doesn't work on recent versions of cmake.

[kroening]

cmake complains "CMake Error: The source directory "" does not exist."
Perhaps simply omit?

[codecov-io]

Codecov Report

Merging #5208 into develop will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop    #5208   +/-   ##
========================================
  Coverage    67.37%   67.37%           
========================================
  Files         1157     1157           
  Lines        95087    95087           
========================================
  Hits         64068    64068           
  Misses       31019    31019

Flag	Coverage Δ
#cproversmt2	42.65% <ø> (ø)	⬆️
#regression	63.89% <ø> (ø)	⬆️
#unit	31.92% <ø> (ø)	⬆️

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[allredj]

✔️
Passed Diffblue compatibility checks (cbmc commit: ac76105).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/142412611

[karkhaz]

Apparently cmake flags are sensitive to whitespace, -S. doesn't work on some platforms but -S . does. What's even weirder is that it doesn't complain about -Bbuild. I have no idea.

Anyway, I fixed the documentation and CI manifests. Thanks.