Link C libraries against build include folder

[bjack205]

Description
This PR fixes a bug in the build system. The lapacke and cblas targets include the source include folder instead of the build include folder. This means that any target downstream target that links against these using target_link_libraries won't get the correct include folder, which critically is missing the auto-generated xxx_mangling.h headers. This PR changes the build interface so that these targets now correctly use the ${LAPACK_BINARY_DIR}/include directory.

This arose from issue #614, which I was trying to build the lapack library after downloading it via FetchContent, which is useful for bundling lapack together with source code rather than relying on a system installation.

I'm not sure where/how to update the documentation for this change, but am happy to do so with some input from the maintainers.

Checklist

• The documentation has been updated.
• If the PR solves a specific issue, it is set to be closed on merge.

[codecov]

Codecov Report

Merging #616 (b488310) into master (afb5242) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #616   +/-   ##
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  Coverage    0.00%    0.00%           
=======================================
  Files        1894     1894           
  Lines      184021   184021           
=======================================
  Misses     184021   184021           

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