Add ability to put different package headers than install path

The SPICE protocol package installs headers in /usr/include/spice-1/spice,
but then returns -I/usr/include/spice-1, the code using #include <spice/blah.h>
So adding one level of indirection to deal with that case.

config.mk

@@ -69,6 +69,11 @@ PACKAGE_DIR?=$(PACKAGE_NAME:%=%/)
 PACKAGE_LIBS=$(MIQ_PRODLIB)
 PACKAGE_DLLS=$(MIQ_PRODDLL)
 
+# Package configuration directories by default
+PACKAGE_PREFIX?=$(PREFIX)
+PACKAGE_PREFIX_LIB?=$(PREFIX_LIB)
+PACKAGE_PREFIX_HDR?=$(PREFIX_HDR)
+
 # Local setup - Location of configuration files, etc (tweaked at install time)
 -include $(MIQ)config.local-setup.mk
 CONFIG_SOURCES?=$(MIQ)config/

rules.mk

@@ -596,10 +596,10 @@ benchmark:	$(BENCHMARKS:%=%.benchmark)
 MIQ_PACKAGELIBS=$(PACKAGE_LIBS:%.lib=$(LINK_LIB_OPT)%)	\
 		$(PACKAGE_DLLS:%.dll=$(LINK_DLL_OPT)%)
 MIQ_GENPC=					  	 \
-	(echo 'prefix=$(PREFIX:%/=%)'			;\
+	(echo 'prefix=$(PACKAGE_PREFIX:%/=%)'		;\
 	echo 'exec_prefix=$${prefix}'			;\
-	echo 'libdir=$(PREFIX_LIB:%/=%)'		;\
-	echo 'includedir=$(PREFIX_HDR:%/=%)'		;\
+	echo 'libdir=$(PACKAGE_PREFIX_LIB:%/=%)'	;\
+	echo 'includedir=$(PACKAGE_PREFIX_HDR:%/=%)'	;\
 	echo 'Name: $(PACKAGE_NAME)'			;\
 	echo 'Description: $(PACKAGE_DESCRIPTION)'	;\
 	echo 'Version: $(PACKAGE_VERSION)'		;\