feat(build): remove timestamps from build to allow reproducible builds

avoid Qt embedding timestamps into translations after copying .ts to .qm to work towards reproducible builds. Now reproducible locally as long as there is absolutely no change to environment.

CMakeLists.txt

@@ -69,6 +69,13 @@ set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wstrict-overflow")
 set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wstrict-aliasing")
 set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Werror")
 
+# avoid timestamps in binary for reproducible builds, not added until GCC 4.9
+include(CheckCXXCompilerFlag)
+CHECK_CXX_COMPILER_FLAG(-Wdate-time COMPILER_SUPPORTS_WDATE_TIME)
+if(COMPILER_SUPPORTS_WDATE_TIME)
+  set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wdate-time")
+endif()
+
 if (NOT WIN32)
   set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -fstack-protector-all")
   set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wstack-protector")
@@ -101,6 +108,15 @@ include_directories(${CMAKE_SOURCE_DIR})
 
 include(Dependencies)
 
+if(NOT Qt5Widgets_VERSION VERSION_LESS "5.9")
+  # Drop the file modification time of source files from generated files
+  # to help with reproducible builds. We do not use QFileInfo.lastModified
+  # so this has no unwanted side effects. This mtime started appearing in
+  # Qt 5.8. The option to force the old file format without mtime was
+  # added in Qt 5.9. See https://bugreports.qt.io/browse/QTBUG-58769
+  set(RCC_OPTIONS ${RCC_OPTIONS} -format-version 1)
+endif()
+
 ################################################################################
 #
 # :: qTox main library sources

cmake/Dependencies.cmake

@@ -199,14 +199,6 @@ add_definitions(
   -DGIT_VERSION="${GIT_VERSION}"
 )
 
-if (NOT TIMESTAMP)
-  execute_process(
-    COMMAND date +%s
-    OUTPUT_VARIABLE TIMESTAMP
-    OUTPUT_STRIP_TRAILING_WHITESPACE
-  )
-endif()
-
 set(APPLE_EXT False)
 if (FOUNDATION_FOUND AND IOKIT_FOUND)
   set(APPLE_EXT True)
@@ -230,6 +222,5 @@ if (PLATFORM_EXTENSIONS)
 endif()
 
 add_definitions(
-  -DTIMESTAMP=${TIMESTAMP}
   -DLOG_TO_FILE=1
 )