cmake: Improve blst build on macOS

[chfast]

Pass the same C compiler which CMake is using to the blst build script and make sure the compiler's required flags (e.g. -isysroot is also included. Previously we relied on the script to find a C compiler on its own, but this might have not worked.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.29%. Comparing base (6974fd7) to head (bb5704f).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #986   +/-   ##
=======================================
  Coverage   94.29%   94.29%           
=======================================
  Files         143      143           
  Lines       16142    16142           
=======================================
  Hits        15221    15221           
  Misses        921      921           

Flag	Coverage Δ
eof_execution_spec_tests	16.67% <ø> (ø)
ethereum_tests	26.58% <ø> (ø)
ethereum_tests_silkpre	18.68% <ø> (ø)
execution_spec_tests	17.72% <ø> (ø)
unittests	89.73% <ø> (ø)

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