configure_file: warn in case a variable to subsitute is not present i…

[lazka]

…n the passed configuration. Fixes #2090

I could't find any unit tests for the affected functions, so no tests atm, ideas where to put them welcome.

[codecov-io]

Codecov Report

Merging #2094 into master will increase coverage by 7.61%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2094      +/-   ##
==========================================
+ Coverage   56.29%   63.91%   +7.61%     
==========================================
  Files          72       72              
  Lines       17164    17172       +8     
  Branches     3536     3537       +1     
==========================================
+ Hits         9663    10976    +1313     
+ Misses       6404     5103    -1301     
+ Partials     1097     1093       -4

Impacted Files	Coverage Δ
mesonbuild/interpreter.py	74.49% <100%> (+0.03%)	⬆️
mesonbuild/mesonlib.py	81.72% <100%> (+0.77%)	⬆️
mesonbuild/dependencies/ui.py	71.14% <0%> (+0.28%)	⬆️
mesonbuild/compilers/fortran.py	69.41% <0%> (+1.76%)	⬆️
mesonbuild/mintro.py	54.73% <0%> (+2.1%)	⬆️
mesonbuild/compilers/compilers.py	79.93% <0%> (+2.15%)	⬆️
mesonbuild/modules/__init__.py	90.14% <0%> (+2.81%)	⬆️
mesonbuild/build.py	76.97% <0%> (+3.61%)	⬆️
mesonbuild/backend/ninjabackend.py	88.15% <0%> (+3.77%)	⬆️
... and 17 more

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[jpakkane]

This is just a FYI warning to users, we don't require tests for those. Thanks.