[all] be smarter about getting the git version

version.cmake

@@ -0,0 +1,34 @@
+execute_process(COMMAND git describe --always --long --abbrev=10 --tags
+	            OUTPUT_VARIABLE GIT_REV
+	            ERROR_QUIET)
+
+if (NOT "${GIT_REV}" STREQUAL "")
+	execute_process(
+		COMMAND bash -c "git diff --quiet --exit-code || echo +"
+		OUTPUT_VARIABLE GIT_DIFF)
+	execute_process(
+		COMMAND git describe --exact-match --tags
+		OUTPUT_VARIABLE GIT_TAG ERROR_QUIET)
+	execute_process(
+		COMMAND git rev-parse --abbrev-ref HEAD
+		OUTPUT_VARIABLE GIT_BRANCH)
+
+	string(STRIP "${GIT_REV}" GIT_REV)
+endif()
+
+set(GIT_VERSION "const char * BUILD_VERSION = \"${GIT_REV}\";")
+
+if(EXISTS ${CMAKE_BINARY_DIR}/version.c)
+	file(READ ${CMAKE_BINARY_DIR}/version.c GIT_VERSION_)
+else()
+	if (EXISTS ${PROJECT_TOP}/VERSION)
+		file(READ ${PROJECT_TOP}/VERSION GIT_VERSION_)
+	else()
+		set(GIT_VERSION_ "UNKNOWN")
+	endif()
+endif()
+
+if (NOT "${GIT_VERSION}" STREQUAL "${GIT_VERSION_}")
+	file(WRITE ${CMAKE_BINARY_DIR}/version.c "${GIT_VERSION}")
+	file(WRITE ${CMAKE_BINARY_DIR}/VERSION   "${GIT_REV}")
+endif()