cmake: use string(TIMESTAMP)

CMakeLists.txt

@@ -30,14 +30,8 @@ add_definitions("${VERSION_GIT_COMMIT_FLAG}")
 SET(VERSION_GIT_BRANCH_FLAG "-DSWAY_GIT_BRANCH=\"${GIT_BRANCH}\"")
 add_definitions("${VERSION_GIT_BRANCH_FLAG}")
 
-execute_process(
+string(TIMESTAMP CURRENT_DATE "%Y-%m-%d" UTC)
-    COMMAND date +"%Y-%m-%d"
+add_definitions("-DSWAY_VERSION_DATE=${CURRENT_DATE}")
-    OUTPUT_VARIABLE CURRENT_DATE
-    OUTPUT_STRIP_TRAILING_WHITESPACE
-)
-
-SET(VERSION_DATE_FLAG "-DSWAY_VERSION_DATE=${CURRENT_DATE}")
-add_definitions("${VERSION_DATE_FLAG}")
 
 find_package(XKBCommon REQUIRED)
 find_package(WLC REQUIRED)