Do not put -lumfpack in LIBS, rather use a special variable for it

mex/build/octave/bytecode/Makefile.am

@@ -2,4 +2,4 @@ EXEEXT = .mex
 include ../mex.am
 include ../../bytecode.am
 
-bytecode_LDADD = -lumfpack
+bytecode_LDADD = $(LIBADD_UMFPACK)

mex/build/octave/configure.ac

@@ -70,7 +70,8 @@ AX_SLICOT([octave])
 AM_CONDITIONAL([HAVE_SLICOT], [test "x$has_slicot" = "xyes"])
 
 # Check for UMFPACK, needed by bytecode
-AC_CHECK_LIB([umfpack], [umfpack_dl_defaults], [], [AC_MSG_ERROR([Can't find UMFPACK])])
+AC_CHECK_LIB([umfpack], [umfpack_dl_defaults], [LIBADD_UMFPACK="-lumfpack"], [AC_MSG_ERROR([Can't find UMFPACK])])
+AC_SUBST([LIBADD_UMFPACK])
 
 AM_CONDITIONAL([DO_SOMETHING], [test "x$MKOCTFILE" != "x"])