configure: add --exec-prefix and fix pkgconfig file paths.

data/Makefile

@@ -5,7 +5,9 @@ PCFILE = libxbps.pc
 
 .PHONY: all
 all:
-	sed -e "s|@@VERSION@@|${VERSION}|g;s|@@PREFIX@@|${PREFIX}|g" \
+	sed -e "s|@@VERSION@@|${VERSION}|g" \
+		-e "s|@@PREFIX@@|${PREFIX}|g" \
+		-e "s|@@EPREFIX@@|${EPREFIX}|g" \
 		${PCFILE}.in > ${PCFILE}
 
 .PHONY: install

data/libxbps.pc.in

@@ -1,10 +1,10 @@
 prefix=@@PREFIX@@
-exec_prefix=${prefix}
+exec_prefix=@@EPREFIX@@
 libdir=${exec_prefix}/lib
-includedir=${prefix}/include
+includedir=${exec_prefix}/include
 
 Name: XBPS API Library
 Description: The X Binary Package System library
 Version: @@VERSION@@
-Libs: -lprop -lxbps
-Cflags: -I${includedir} -L${libdir}
+Libs: -lprop -lxbps -L${libdir}
+Cflags: -I${includedir}