make most vars immediate

Makefile

@@ -1,23 +1,23 @@
-DATADIR  = data
-TMPDIR   = _tmp
-BUILDDIR = _build
-INFONAME = info.yaml
-ROOT     = https://gallery.niss.website
+DATADIR  := data
+TMPDIR   := _tmp
+BUILDDIR := _build
+INFONAME := info.yaml
+ROOT     := https://gallery.niss.website
 
 # SMALL = thumbnails, MED = single pages (link to full size)
 SMALL := 200
 MEDW  := 1000
 MEDH  := 1200
 
-MAKEPAGES = $(TMPDIR)/make-pages
+MAKEPAGES := $(TMPDIR)/make-pages
 
 YAMLS != find $(DATADIR) -name $(INFONAME)
 
 SCRIPTS := $(wildcard script/*.js)
 
 STYLES    := $(wildcard style/*.css)
-STYLESVGS = $(wildcard style/*.svg)
-STYLEPNGS = $(wildcard style/*.png)
+STYLESVGS := $(wildcard style/*.svg)
+STYLEPNGS := $(wildcard style/*.png)
 
 FONTS := $(shell find fonts \
   -iname '*.eot'   -or -iname '*.svg'  -or \