Remove non-include paths from INC in pre-build checks

[paultcochrane]

While trying to get PDL::Graphics::PLplot to install, I found it kept telling me that it needed a plplot library compiled with the --with-double option:

$ perl Makefile.PL
Use of uninitialized value $size in numeric eq (==) at Makefile.PL line 46.
Sizeof(PLFLT) must be 8.
PLplot must be compiled --with-double (IE ./configure --with-double).

Please either:
- Install PLplot using your package manager then reinstall Alien::PLplot
  or
- Reinstall Alien::PLplot from CPAN with environment variable
  ALIEN_INSTALL_TYPE=share.

After having tried the advice about reinstalling Alien::PLplot and failing, and having checked that libplplot had been installed using double precision, I happened to stumble upon the issue that the option -pthread was appearing in the $plplot_include_path variable. I noticed this by replacing the call to check_lib (from Devel::CheckLib) with check_lib_or_exit. When using this function, the error output was this instead:

$ perl Makefile.PL
INC argument badly-formed: -pthread

Upon reading the docs for Devel::CheckLib, it turns out that the INC option to both check_lib and check_lib_or_exit needs to be a space-delimited list of options, all of which start with -I. Since -pthread doesn't start with -I the argument was badly formed, hence the analyze_binary callback was never called. Thus no check code was compiled which then resulted in $size being set to undef. Because $size was undef, the uninitialized value warning mentioned in the first error message mentione above and the check for a double precision library failed. In other words, because -pthread was turning up in the $plplot_include_path string, the test for a library compiled with the --with-double option was failing and hence PDL::Graphics::PLplot failed to install.

By filtering only for include paths from the list of options originally found in $plplot_include_path (i.e. selecting only options starting with -I and hence excluding in this particular instance the -pthread option), one avoids the issue of a badly formed INC argument and thus the checks in Makefile.PL compile and run. Because all checks run, the Makefile is created as per normal and the test suite passes.

More background info: I found this issue on Debian version 11.11 with a perlbrewed perl, version 5.32.1.

This PR is submitted in the hope that it is useful. If you'd like anything changed, please let me know and I'll update and resubmit as necessary.