pkg-config variables for HiGHS

[Michael Orlitzky]

Hello, we are working on adding HiGHS to SageMath, and while reviewing
the PR, I noticed some workarounds in the meson file to detect
libhighs. They are apparently due to the following in highs.pc from
the libhighs-dev (1.12.0+ds1-1) package:

  prefix=/usr
  libdir=/usr/debian/tmp/usr/lib/x86_64-linux-gnu
  includedir=/usr/debian/tmp/usr/include/highs

I think that is simply a bug? If so, it will take a little while to
get the HiGHS PR merged -- we can probably forego the hacks and wait
for a fix.

Thanks (and happy holidays).