Use version from VERSION file if git describe fails

This fixes issue #100.

We prefer to determine version from git tags (using git describe).
However, sometimes people don't use git or don't have it installed,
which leads to empty version used.
To solve this, we use version from VERSION file as a backup.

Makefile

@@ -15,6 +15,9 @@ CC ?= gcc
 CFLAGS ?= -g -O0
 CFLAGS += -Wall -Wextra -std=c99 -pedantic
 GIT_VERSION := $(shell git describe --abbrev=4 --dirty --always --tags)
+ifeq ($(GIT_VERSION),)
+    GIT_VERSION := $(shell cat VERSION)
+endif
 CFLAGS += -DPROGRAM_VERSION=\"$(GIT_VERSION)\"
 
 ifeq ($(UNAME_S),Linux)

VERSION

@@ -0,0 +1 @@
+v2.0.0-dev