set ilp64 to an invalid kind if not enabled

Author: perazz

src/stdlib_linalg_constants.fypp

@@ -29,13 +29,13 @@ module stdlib_linalg_constants
      logical(lk), parameter :: external_lapack_ilp64 = .false._lk
 #endif   
 
-     ! Generic checks
+     ! Generic checks for external libraries
      logical(lk), parameter :: external_blas   = external_blas_ilp32   .or. external_blas_ilp64
      logical(lk), parameter :: external_lapack = external_lapack_ilp32 .or. external_lapack_ilp64
 
      ! Support both 32-bit (ilp) and 64-bit (ilp64) integer kinds
      integer, parameter :: ilp   = int32
-     integer, parameter :: ilp64 = int64
+     integer, parameter :: ilp64 = #{if WITH_ILP64}# int64 #{else}# -1 #{endif}#
      private            :: int32, int64
 
 end module stdlib_linalg_constants