use cmake TIMESTAMP

to make build more platform-independent
and to allow to override build date to enable reproducible builds

See https://reproducible-builds.org/ for why this is good.

Also use UTC to be independent of timezones.

CMakeLists.txt

@@ -1,4 +1,4 @@
-cmake_minimum_required(VERSION 2.8)
+cmake_minimum_required(VERSION 2.8.11)
 set(CMAKE_LEGACY_CYGWIN_WIN32 0)
 
 project(Cadabra)
@@ -17,7 +17,7 @@ endif()
 if(GIT_COMMIT_DATE)
   set(CADABRA_VERSION_DATE "${GIT_COMMIT_DATE}")
 else()
-  execute(COMMAND date -I OUTPUT_VARIABLE THE_DATE OUTPUT_STRIP_TRAILING_WHITESPACE)
+  string(TIMESTAMP THE_DATE "%Y-%m-%d" UTC)
   set(CADABRA_VERSION_DATE "${THE_DATE}") 
 endif()
 message("-- Building ${CADABRA_VERSION_MAJOR}.${CADABRA_VERSION_MINOR}.${CADABRA_VERSION_PATCH}")