Index: Makefile.in
==================================================================
--- Makefile.in
+++ Makefile.in
@@ -1,10 +1,13 @@
+monocypher_dir := @srcdir@/vendor/monocypher/
+argon2_dir     := @srcdir@/vendor/argon2/
+
 CC            := @CC@
 AR            := @AR@
 RANLIB        := @RANLIB@
 CFLAGS        := @CFLAGS@ @SHOBJFLAGS@
-CPPFLAGS      := -I. -I@srcdir@ -I@srcdir@/vendor/monocypher/ -I@srcdir@/vendor/argon2/ @CPPFLAGS@ @SHOBJCPPFLAGS@ @DEFS@ @TCL_DEFS@
+CPPFLAGS      := -I. -I@srcdir@ -I$(monocypher_dir) -I$(argon2_dir) @CPPFLAGS@ @SHOBJCPPFLAGS@ @DEFS@ @TCL_DEFS@
 LDFLAGS       := @LDFLAGS@
 LIBS          := @LIBS@
 SHOBJLDFLAGS  := @SHOBJLDFLAGS@
 VPATH         := @srcdir@
 srcdir        := @srcdir@
@@ -34,28 +37,28 @@
 	-$(RANLIB) @EXTENSION_TARGET@
 endif
 
 # The amalgamation is used when compiling statically so that the same ABI can be exposed
 # to upstream projects rather than requiring them to filter out our symbols
-nano-amalgamation.c: @srcdir@/nano.c @srcdir@/randombytes.c @srcdir@/vendor/monocypher/monocypher.c @srcdir@/argon2.c Makefile
+nano-amalgamation.c: @srcdir@/nano.c @srcdir@/randombytes.c $(monocypher_dir)monocypher.c $(argon2_dir)argon2.c Makefile
 	rm -f nano-amalgamation.c
-	cat @srcdir@/nano.c @srcdir@/randombytes.c @srcdir@/vendor/monocypher/monocypher.c @srcdir@/vendor/argon2/argon2.c > nano-amalgamation.c
+	cat @srcdir@/nano.c @srcdir@/randombytes.c $(monocypher_dir)monocypher.c $(argon2_dir)argon2.c > nano-amalgamation.c
 
-nano-amalgamation.o: nano-amalgamation.c @srcdir@/vendor/monocypher/monocypher.h @srcdir@/vendor/argon2/argon2.h @srcdir@/randombytes.h nano.tcl.h Makefile
+nano-amalgamation.o: nano-amalgamation.c $(monocypher_dir)monocypher.h $(argon2_dir)argon2.h @srcdir@/randombytes.h nano.tcl.h Makefile
 	$(CC) $(CPPFLAGS) $(CFLAGS) -o nano-amalgamation.o -c nano-amalgamation.c
 
-nano.o: @srcdir@/nano.c @srcdir@/vendor/monocypher/monocypher.h @srcdir@/vendor/argon2/argon2.h @srcdir@/randombytes.h nano.tcl.h Makefile
+nano.o: @srcdir@/nano.c $(monocypher_dir)monocypher.h $(argon2_dir)argon2.h @srcdir@/randombytes.h nano.tcl.h Makefile
 	$(CC) $(CPPFLAGS) $(CFLAGS) -o nano.o -c @srcdir@/nano.c
 
 randombytes.o: @srcdir@/randombytes.c @srcdir@/randombytes.h
 	$(CC) $(CPPFLAGS) $(CFLAGS) -o randombytes.o -c @srcdir@/randombytes.c
 
-monocypher.o: @srcdir@/vendor/monocypher/monocypher.c @srcdir@/vendor/monocypher/monocypher.h
-	$(CC) $(CPPFLAGS) $(CFLAGS) -o monocypher.o -c @srcdir@/vendor/monocypher/monocypher.c
+monocypher.o: $(monocypher_dir)monocypher.c $(monocypher_dir)monocypher.h
+	$(CC) $(CPPFLAGS) $(CFLAGS) -o monocypher.o -c $(monocypher_dir)monocypher.c
 
-argon2.o: @srcdir@/vendor/argon2/argon2.c @srcdir@/vendor/argon2/argon2.h
-	$(CC) $(CPPFLAGS) $(CFLAGS) -o argon2.o -c @srcdir@/vendor/argon2/argon2.c
+argon2.o: $(argon2_dir)argon2.c $(argon2_dir)argon2.h
+	$(CC) $(CPPFLAGS) $(CFLAGS) -o argon2.o -c $(argon2_dir)argon2.c
 
 pkgIndex.tcl: pkgIndex.tcl-@TCLEXT_BUILD@
 	cp pkgIndex.tcl-@TCLEXT_BUILD@ pkgIndex.tcl
 
 nano.tcl.h: @srcdir@/nano.tcl Makefile
@@ -98,9 +101,9 @@
 	rm -rf coverage.dir
 
 mrproper: distclean
 	rm -f @srcdir@/configure @srcdir@/aclocal.m4 @srcdir@/config.guess @srcdir@/config.sub @srcdir@/install-sh
 	rm -f @srcdir@/nano.vers
-	rm -rf @srcdir@/vendor/argon2
+	rm -rf '$(argon2_dir)'
 	$(MAKE) -C @srcdir@/build/argon2 distclean
 
 .PHONY: all test clean distclean mrproper