diff --git a/src/Makefile b/src/Makefile
index 30ba106..f7f6827 100644
--- a/src/Makefile
+++ b/src/Makefile
@@ -1,6 +1,8 @@
 DESTDIR ?= /usr/local
 CFLAGS = $(shell pkg-config --cflags guile-2.0) -fPIC
 LDFLAGS = $(shell pkg-config --libs guile-2.0)
+EXTDIR = $(shell pkg-config guile-2.0 --variable=extensiondir \
+	--define-variable=prefix=$(DESTDIR))
 
 objects = gitto-path.o
 
@@ -18,10 +20,9 @@ libguile-gitto-path.so: gitto-path.o
 install-gitto: gitto
 	install -Dm 755 $^ ${DESTDIR}/bin/$^
 install-libguile-gitto-path: libguile-gitto-path.so
-	install -Dm 755 $^ \
-		$(shell pkg-config guile-2.0 --variable=extensiondir)/$^
+	install -Dm 755 $^ ${EXTDIR}/$^
 install: install-gitto install-libguile-gitto-path
 
 uninstall:
 	rm -f ${DESTDIR}/bin/gitto
-	rm -f $(shell pkg-config guile-2.0 --variable=extensiondir)/libguile-gitto-path.so
+	rm -f ${EXTDIR}/libguile-gitto-path.so