fix default prefix handling in makefiles

PREFIX wasn't communicated to Makefile.install (where it matters)
by Makefile. The result is that the default PREFIX is empty and
OpenBLAS was being installed in /lib.

Makefile

@@ -15,10 +15,6 @@ ifdef SANITY_CHECK
 BLASDIRS += reference
 endif
 
-ifndef PREFIX
-PREFIX = /opt/OpenBLAS
-endif
-
 SUBDIRS	= $(BLASDIRS)
 ifneq ($(NO_LAPACK), 1)
 SUBDIRS	+= lapack

Makefile.install

@@ -3,6 +3,8 @@ export GOTOBLAS_MAKEFILE = 1
 -include $(TOPDIR)/Makefile.conf_last
 include ./Makefile.system
 
+PREFIX?=/opt/OpenBLAS
+
 OPENBLAS_INCLUDE_DIR:=$(PREFIX)/include
 OPENBLAS_LIBRARY_DIR:=$(PREFIX)/lib
 OPENBLAS_BUILD_DIR:=$(CURDIR)