Use SOURCE_DATE_EPOCH for timestamp if present

The Debian reproducible builds effort suggests using
this environment variable for timestamps.

CMakeLists.txt

@@ -14,8 +14,6 @@ set (HS_VERSION ${HS_MAJOR_VERSION}.${HS_MINOR_VERSION}.${HS_PATCH_VERSION})
 set (CMAKE_CONFIGURATION_TYPES "Debug;Release;RelWithDebInfo"
      CACHE STRING "" FORCE)
 
-string (TIMESTAMP BUILD_DATE "%Y-%m-%d")
-
 set(CMAKE_MODULE_PATH ${PROJECT_SOURCE_DIR}/cmake)
 include(CheckCCompilerFlag)
 include(CheckCXXCompilerFlag)
@@ -98,6 +96,18 @@ else()
     message(FATAL_ERROR "No python interpreter found")
 endif()
 
+# allow for reproducible builds - python for portability
+if (DEFINED ENV{SOURCE_DATE_EPOCH})
+      execute_process(
+          COMMAND "${PYTHON}" "${CMAKE_MODULE_PATH}/formatdate.py" "$ENV{SOURCE_DATE_EPOCH}"
+          OUTPUT_VARIABLE BUILD_DATE
+          OUTPUT_STRIP_TRAILING_WHITESPACE)
+else ()
+    string (TIMESTAMP BUILD_DATE "%Y-%m-%d")
+endif ()
+message(STATUS "Build date: ${BUILD_DATE}")
+
+
 if(${RAGEL} STREQUAL "RAGEL-NOTFOUND")
     message(FATAL_ERROR "Ragel state machine compiler not found")
 endif()