fix library names in lapack64.pc

Add substitutions to `lapack.pc.in`, in order to ensure that
the generated `lapack64.pc` file correctly references the index64
library and BLAS dependency.  It seems that other pkg-config templates
have been updated as part of #462 but this one was omitted.
As a result, finding a lapack64 via pkg-config gave non-index64
libraries.

Signed-off-by: Michał Górny <[email protected]>

Author: mgorny

lapack.pc.in

@@ -5,5 +5,5 @@ Name: LAPACK
 Description: FORTRAN reference implementation of LAPACK Linear Algebra PACKage
 Version: @LAPACK_VERSION@
 URL: http://www.netlib.org/lapack/
-Libs: -L${libdir} -llapack
-Requires.private: blas
+Libs: -L${libdir} -l@LAPACKLIB@
+Requires.private: @BLASLIB@