Meson: handle backshash chars in strings and == operator correctly

[masatake]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 85.84%. Comparing base (f3f720e) to head (f786a70).
Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
optlib/meson.c	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4104      +/-   ##
==========================================
- Coverage   85.84%   85.84%   -0.01%     
==========================================
  Files         239      239              
  Lines       58602    58603       +1     
==========================================
  Hits        50306    50306              
- Misses       8296     8297       +1     

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