dependencies: Always force PKG_CONFIG_ALLOW_SYSTEM_CFLAGS

[amyspark]

This is needed too for pkgconf because it strips them by default when built with Meson. The autotools alternatives provides an option to skip that behaviour, but it would be best not to rely on build semantics.

Fixes #11088

[codecov[bot]]

Codecov Report

Merging #11124 (9f639d0) into master (212af2b) will decrease coverage by 1.28%. The diff coverage is n/a.

:exclamation: Current head 9f639d0 differs from pull request most recent head 69990fc. Consider uploading reports for the commit 69990fc to get more accurate results

@@            Coverage Diff             @@
##           master   #11124      +/-   ##
==========================================
- Coverage   68.61%   67.32%   -1.29%     
==========================================
  Files         412      207     -205     
  Lines       87760    44952   -42808     
  Branches    19377     9294   -10083     
==========================================
- Hits        60213    30263   -29950     
+ Misses      23045    12406   -10639     
+ Partials     4502     2283    -2219     

Impacted Files	Coverage Δ
modules/cuda.py	0.00% <0.00%> (-72.64%)	:arrow_down:
templates/cudatemplates.py	37.50% <0.00%> (-62.50%)	:arrow_down:
compilers/cuda.py	19.63% <0.00%> (-45.40%)	:arrow_down:
dependencies/cuda.py	20.19% <0.00%> (-42.79%)	:arrow_down:
compilers/mixins/clang.py	52.87% <0.00%> (-13.80%)	:arrow_down:
scripts/coverage.py	56.43% <0.00%> (-7.93%)	:arrow_down:
compilers/detect.py	42.97% <0.00%> (-3.99%)	:arrow_down:
linkers/linkers.py	56.35% <0.00%> (-1.25%)	:arrow_down:
modules/qt.py	74.28% <0.00%> (-1.17%)	:arrow_down:
environment.py	80.12% <0.00%> (-1.08%)	:arrow_down:
... and 250 more

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[eli-schwartz]

This seems fundamentally problematic, because the system includedir shouldn't need to be manually specified in build.ninja's compile args, and after this change it is manually specified.