Meson: extract config vars defined inside configuration_data({...})

[masatake]

I should pull this request into two.

(
This random should be removed from here:

input : "\n" L3781
match :'\n'            main[ 0] / ^([a-zA-Z_][a-zA-Z_0-9]*)[ \t\n]*=([^=]|$)/
match :'\n'            main[ 1] / ^project[ \t\n]*\\([ \t\n]*'([^']*[^\\\\])'[ \t\n]*/
match :'\n'            main[ 2] / ^subdir[ \t\n]*\\([ \t\n]*'([^']*[^\\\\])'[ \t\n]*/
match :'\n'            main[ 3] / ^test[ \t\n]*\\([ \t\n]*'([^']*[^\\\\])'[ \t\n]*/
match :'\n'            main[ 4] / ^[ \t\n]+/

...                   TABLE[ N] ...
result: matched 1 bytes
scope : 
action: NOP in {main}, stack: /

It would be helpful for optimization if ctags could report per-TABLE statistics (average N, s.d. N, max N).
)

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 64 lines in your changes missing coverage. Please review.

Project coverage is 85.89%. Comparing base (4fcf455) to head (bcbc81d).
Report is 5 commits behind head on master.

Files with missing lines	Patch %	Lines
optlib/meson.c	0.00%	64 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4275      +/-   ##
==========================================
- Coverage   85.96%   85.89%   -0.08%     
==========================================
  Files         246      246              
  Lines       63454    63517      +63     
==========================================
+ Hits        54546    54555       +9     
- Misses       8908     8962      +54     

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