BLD: fix MinGW Windows build issue by splitting linalg.interpolate sources.

Type of errors that this fixes::

    Argument #5 (named `ifac') of `zfftb1' is one type at (2) but is some
    other type at (1) [info -f g77 M GLOBALS]

Warning shows up with gfortran as::

    Warning: Type mismatch in argument 'ifac' at (1); passed REAL(8) to INTEGER(4)

Closes gh-2709.
BLD: ignore split Fortran source files created by the build process.

Also add some other generated files to .gitignore
BLD: use relative paths in linalg/setup.py
MAINT: address review comments on splitting Fortran files at build time.
BLD: exclude split Fortran files from sdist generated source bundles.
BLD: recognize one more Fortran comment character
Merge pull request #2761 from rgommers/fix-mingw-compilation

BLD: Fix MinGW compilation by splitting source files

Fixes gh-2709