Use $(includedir) instead of hardcoding $(prefix)/include

Make.inc

@@ -6,6 +6,7 @@ VERSION_SPLIT = $(subst ., , $(VERSION))
 DESTDIR =
 prefix = /usr/local
 libdir = $(prefix)/lib
+includedir = $(prefix)/include
 
 FC = gfortran
 FFLAGS += -O3

Makefile

@@ -43,6 +43,6 @@ distclean:
 
 install: all
 	mkdir -p $(DESTDIR)$(libdir)
-	mkdir -p $(DESTDIR)$(prefix)/include
+	mkdir -p $(DESTDIR)$(includedir)
 	cp -a libopenlibm.$(SHLIB_EXT)* libopenlibm.a $(DESTDIR)$(libdir)/
-	cp -a src/openlibm.h $(DESTDIR)$(prefix)/include/
+	cp -a src/openlibm.h $(DESTDIR)$(includedir)/