Make package release dates reproducible

Using `string(TIMESTAMP ...)` respects `SOURCE_DATE_EPOCH` since CMake 3.8, which makes it possible to fix the date used as release date in AppStream metadata and Fedora packaging.

CMakeLists.txt

@@ -19,8 +19,8 @@ SET(INTERFACE_TYPE "qt5" CACHE STRING "Select interface type: qt5/qt6/gtk")
 MESSAGE(STATUS "${INTERFACE_TYPE} interface will be built")
 SET(MANUAL_DIR "share/doc/gimagereader" CACHE PATH "Path where manual will be installed")
 SET(ENABLE_VERSIONCHECK 1 CACHE BOOL "Enable version check")
-EXECUTE_PROCESS(COMMAND date +%a\ %b\ %d\ %Y OUTPUT_VARIABLE PACKAGE_DATE OUTPUT_STRIP_TRAILING_WHITESPACE)
-EXECUTE_PROCESS(COMMAND date +%Y-%m-%d OUTPUT_VARIABLE PACKAGE_ISO_DATE OUTPUT_STRIP_TRAILING_WHITESPACE)
+string(TIMESTAMP PACKAGE_DATE "%a\ %b\ %d\ %Y" UTC)
+string(TIMESTAMP PACKAGE_ISO_DATE "%Y-%m-%d" UTC)
 EXECUTE_PROCESS(COMMAND date -R OUTPUT_VARIABLE PACKAGE_RFC_DATE OUTPUT_STRIP_TRAILING_WHITESPACE)
 EXECUTE_PROCESS(COMMAND git rev-parse HEAD OUTPUT_VARIABLE PACKAGE_REVISION OUTPUT_STRIP_TRAILING_WHITESPACE)