Freshen version number when building with CMake

Fixes #979

src/.gitignore

@@ -0,0 +1,2 @@
+# Generated by CMake
+/.version.c

src/CMakeLists.txt

@@ -6,10 +6,12 @@
 # SPDX-License-Identifier: MIT
 #
 
+configure_file(version.c _version.c ESCAPE_QUOTES)
+
 set(common_src
     "error.c"
     "extern/getopt.c"
-    "version.c"
+    "_version.c"
     )
 
 find_package(PkgConfig)

src/version.c

@@ -12,6 +12,13 @@
 #include "helpers.h"
 #include "version.h"
 
+// This variable is passed via `-D` from the Makefile, but not from CMake
+// (in which `configure_file()` is used on this file to replace some syntax)
+#ifndef BUILD_VERSION_STRING
+// CMake-specific syntax here
+#define BUILD_VERSION_STRING "@GIT_REV@"
+#endif
+
 char const *get_package_version_string(void)
 {
 	// The following conditional should be simplified by the compiler.