[WIP] Add a settings file to configure cobc at runtime

[lefessan]

New --settings=FILE can be used to configure flags that are normally set at compile time. These settings can be used typically to create cross-compilers by retargetting an existing compiler.

Also add a setting to make the compiler relocatable, i.e. compute paths of include, share, etc. relative to cobc's location.

[codecov-commenter]

Codecov Report

Merging #117 (ebc7f78) into gcos4gnucobol-3.x (c0d64ad) will decrease coverage by 0.25%. The diff coverage is 19.44%.

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@@                  Coverage Diff                  @@
##           gcos4gnucobol-3.x     #117      +/-   ##
=====================================================
- Coverage              65.74%   65.49%   -0.25%     
=====================================================
  Files                     32       34       +2     
  Lines                  59092    59345     +253     
  Branches               15575    15679     +104     
=====================================================
+ Hits                   38849    38869      +20     
- Misses                 14262    14457     +195     
- Partials                5981     6019      +38     

Files	Coverage Δ
cobc/typeck.c	65.28% <ø> (ø)
cobc/ppparse.y	61.85% <0.00%> (-0.12%)	:arrow_down:
cobc/cobc.c	72.02% <41.66%> (-0.37%)	:arrow_down:
cobc/settings.def	0.00% <0.00%> (ø)
cobc/codegen.c	75.09% <19.81%> (-0.90%)	:arrow_down:
cobc/settings.c	17.59% <17.59%> (ø)

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