Allow finding TPLs by environment variables

[masterleinad]

Also provide an error message in case AmgX was not found.

[codecov-io]

Codecov Report

Merging #149 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #149   +/-   ##
=======================================
  Coverage   87.76%   87.76%           
=======================================
  Files          53       53           
  Lines        2199     2199           
=======================================
  Hits         1930     1930           
  Misses        269      269

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[Rombur]

Let's not merge this. The build system and this part in particular is very brittle and it is too important to start making change to it at this time in the project. Passing the jenkins is not enough, we need to try on different machines.

[dalg24]

Let's not merge this. The build system and this part in particular is very brittle and it is too important to start making change to it at this time in the project. Passing the jenkins is not enough, we need to try on different machines.

This patch is armless and gives cleaner code in CMake.

[dalg24]

We can strip the part with the env variables if you don't like but this is the correct way to detect the AmgX lib and link against it.

[Rombur]

We shouldn't merge anything in the build system until we know it works on Docker, my machine, and Andrey's.