* PGI C++ PCH enable
PGI compilers support precompiled headers for C++ only.
The common/13 pch test passes if run manually with no spaces in the build path.
However, since Meson run_project_tests.py makes temporary build directories
with spaces in each tests, PGI --pch_dir can't handle this and fails.
So we skip the test for PGI despite it working for usual case with no-spaces
in build dir.
Note: it's fine to have spaces in full path for sourcedir, just no spaces in
relative path to builddir.
* doc