do not hardcode lapack library name

Use @LAPACKLIB@ instead hardcodes -llapack to generate correc
lapack64.pc file with -llapack64.

Signed-off-by: Tomasz Kłoczko <kloczek@github.com>

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${libdir} -llapack
+Libs: -L${libdir} -l@LAPACKLIB@
 Requires.private: blas