Allow snek-install.defs values to be overridden in the environment

Signed-off-by: Keith Packard <keithp@keithp.com>

snek-install.defs

@@ -1,15 +1,15 @@
 DESTDIR =
-PREFIX = /usr/local
-LIBDIR = $(PREFIX)/lib
-BINDIR = $(PREFIX)/bin
-SHAREDIR = $(PREFIX)/share/snek
-DOCDIR = $(PREFIX)/share/doc/snek
-PKGCONFIG = $(LIBDIR)/pkgconfig
-APPDIR = $(PREFIX)/share/applications
-ICONDIR = $(PREFIX)/share/icons/hicolor/scalable/apps
-MANDIR = $(PREFIX)/share/man
-SNEKLIB = $(PREFIX)/share/snek
-SNEKDIST = keithp.com:/var/www/snek/dist
-SNEKDOC = keithp.com:/var/www/snek
+PREFIX ?= /usr/local
+LIBDIR ?= $(PREFIX)/lib
+BINDIR ?= $(PREFIX)/bin
+SHAREDIR ?= $(PREFIX)/share/snek
+DOCDIR ?= $(PREFIX)/share/doc/snek
+PKGCONFIG ?= $(LIBDIR)/pkgconfig
+APPDIR ?= $(PREFIX)/share/applications
+ICONDIR ?= $(PREFIX)/share/icons/hicolor/scalable/apps
+MANDIR ?= $(PREFIX)/share/man
+SNEKLIB ?= $(PREFIX)/share/snek
+SNEKDIST ?= keithp.com:/var/www/snek/dist
+SNEKDOC ?= keithp.com:/var/www/snek
 
 include $(SNEK_ROOT)/snek.defs

snek.defs

@@ -1,6 +1,4 @@
-ifndef SNEK_LOCAL_VPATH
-SNEK_LOCAL_VPATH = .
-endif
+SNEK_LOCAL_VPATH ?= .
 
 vpath %.c $(SNEK_LOCAL_VPATH):$(SNEK_ROOT)
 vpath %.h $(SNEK_LOCAL_VPATH):$(SNEK_ROOT)
@@ -58,7 +56,7 @@ SNEK_WARNINGS = \
 	-Warray-bounds \
 	-Wshadow
 
-SNEKLIB=$(SNEK_ROOT)
+SNEKLIB = $(SNEK_ROOT)
 
 SNEK_CFLAGS = -D_DEFAULT_SOURCE -I$(SNEK_ROOT) $(SNEK_WARNINGS) -DSNEK_VERSION='"$(SNEK_VERSION)"'