Make compiler_flags setting take precedence over the compiler flag env var

[MalachiTimothyPhillips]

Closes #621

[codecov[bot]]

Codecov Report

Merging #622 (01c1f6f) into main (2e576e6) will increase coverage by 0.00%. The diff coverage is 100.00%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #622   +/-   ##
=======================================
  Coverage   77.31%   77.32%           
=======================================
  Files         265      265           
  Lines       19592    19594    +2     
=======================================
+ Hits        15148    15151    +3     
+ Misses       4444     4443    -1     

Impacted Files	Coverage Δ
src/occa/internal/modes/serial/device.cpp	90.27% <100.00%> (+0.54%)	:arrow_up:
src/occa/internal/utils/sys.cpp	77.36% <0.00%> (+0.10%)	:arrow_up:

[MalachiTimothyPhillips]

For consistency, the flags for the SYCL backend should also be updated also.

https://github.com/libocca/occa/blob/2e576e69e07627a516b568079e047965a9bd8d24/src/occa/internal/modes/dpcpp/utils.cpp#L43

(Currently Metal and OpenMP don't have their own env variables for compiler flags).

Done. If we go this route, we should also do #623 .

[kris-rowe]

@MalachiTimothyPhillips can you set your PR to merge into development.

[MalachiTimothyPhillips]

@MalachiTimothyPhillips can you set your PR to merge into development.

Done. Please let me know if you need anything else. Cheers!