check for lessc executable in any location

previously the checks were explicit locations which does not work when
using something like nvm to manage node packages and versions.

this checks for the executable and sets the script variable LESSC to the
full path of the one found.
if none was found the make command will error.

less/Makefile

@@ -1,13 +1,8 @@
-ifeq ($(shell which lessc),/usr/bin/lessc)
-	LESSC=/usr/bin/lessc
-else ifeq ($(shell which lessc),/usr/local/bin/lessc)
-	LESSC=/usr/local/bin/lessc
-else ifeq ($(shell which lessc),/bin/lessc)
-	LESSC=/bin/lessc
-else
-	LESSC=node_modules/.bin/lessc
+LESSC := $(shell command -v lessc 2> /dev/null)
+
+ifndef LESSC
+	$(error "less is not installed, please run install-less.sh")
 endif
-export LESSC
 
 CSSDIR=../static/css/