[Lang][Preprocessor] Adds okl/strict_headers option

[dmed256]

Description

Adds the option to include non-standard system headers

> occa translate addVectors.okl
addVectors.okl:1:2: Error: File does not exist
#include "foobar"
 ^

Adding

kernelProps["okl/strict_headers"] = false;

or passing it in the CLI ends up including headers even if OCCA can't find them:

> occa translate -k "okl: { strict_headers: false }" addVectors.okl
...
#include "foobar"
...

[codecov]

Codecov Report

Merging #408 into main will increase coverage by 0.03%.
The diff coverage is 87.17%.

@@            Coverage Diff             @@
##             main     #408      +/-   ##
==========================================
+ Coverage   76.34%   76.37%   +0.03%     
==========================================
  Files         239      239              
  Lines       18396    18413      +17     
==========================================
+ Hits        14044    14063      +19     
+ Misses       4352     4350       -2     

Impacted Files	Coverage Δ
src/lang/tokenizer.cpp	80.41% <80.76%> (+0.26%)	⬆️
src/lang/preprocessor.cpp	80.40% <100.00%> (+0.18%)	⬆️
src/tools/sys.cpp	65.31% <0.00%> (+0.16%)	⬆️
src/lang/specialMacros.cpp	60.34% <0.00%> (+1.72%)	⬆️