Use an already existing K_HOME environment variable. (#36)

* find-k.mak: Accept an already existing K_HOME variable

* Use `which kompile` to set K_HOME as last resort.

* More explicit path resolution

find-k.mak

@@ -1,12 +1,15 @@
 MAKEFILE_PATH := $(dir $(abspath $(lastword $(MAKEFILE_LIST))))
-ifneq (,$(wildcard ${MAKEFILE_PATH}/../../../include/kframework/ktest.mak))
+ifneq (,${K_HOME})
+else ifneq (,$(wildcard ${MAKEFILE_PATH}/../../../include/kframework/ktest.mak))
 export K_HOME?=${MAKEFILE_PATH}/../../../
 else ifneq (,$(wildcard ${MAKEFILE_PATH}/../include/kframework/ktest.mak))
 export K_HOME?=${MAKEFILE_PATH}/..
 else ifneq (,$(wildcard /usr/include/kframework/ktest.mak))
 export K_HOME?=/usr
 else ifneq (,$(wildcard /usr/local/include/kframework/ktest.mak))
 export K_HOME?=/usr/local
+else ifneq (,$(shell which kompile))
+export K_HOME?=$(abspath $(dir $(shell which kompile))/..)
 else
 $(error "Could not find installation of K. Please set K_HOME environment variable to your K installation.")
 endif