cmake: Make builds fully reproducible by default

- Work-around SOURCE flags being reset for each project when setting the
  random-seed property by calling a support function.
- Likewise, set correct flags for reproducibility when creating archives.
- Strip source prefix paths from debug info

This should set the EPOCH using the last git commit date when available,
but it's not currently implemented. Just set the EPOCH to be 0 for now.

CMakeLists.txt

@@ -184,12 +184,6 @@ SET(FW_SOURCES
 )
 list(TRANSFORM FW_SOURCES PREPEND ${CMAKE_CURRENT_SOURCE_DIR}/Firmware/)
 
-foreach(_FILE ${FW_SOURCES})
-    get_filename_component(_BASE ${_FILE} NAME)
-    set_property(SOURCE ${_FILE} APPEND_STRING PROPERTY COMPILE_FLAGS "-frandom-seed=${_BASE}.o")
-endforeach()
-
-
 set(AVR_SOURCES
     wiring_digital.c
     WInterrupts.c
@@ -218,10 +212,34 @@ set(AVR_SOURCES
 )
 list(TRANSFORM AVR_SOURCES PREPEND ${PRUSA_BOARDS_DIR}/cores/prusa_einsy_rambo/)
 
-foreach(_FILE ${AVR_SOURCES})
-    get_filename_component(_BASE ${_FILE} NAME)
-    set_property(SOURCE ${_FILE} APPEND_STRING PROPERTY COMPILE_FLAGS "-frandom-seed=core/${_BASE}.o")
-endforeach()
+
+#
+# Reproducible build support
+#
+
+function(set_reproducible_sources source_list prefix)
+    foreach(file IN LISTS ${source_list})
+        get_filename_component(base ${file} NAME)
+        set(target "${prefix}${base}")
+        set_property(SOURCE ${file} APPEND PROPERTY COMPILE_OPTIONS "-frandom-seed=${target}.o")
+    endforeach()
+endfunction()
+
+function(set_reproducible_target target)
+    set_target_properties(${target} PROPERTIES STATIC_LIBRARY_OPTIONS "-D")
+endfunction()
+
+set_reproducible_sources(AVR_SOURCES "core/")
+
+add_link_options(-fdebug-prefix-map=${CMAKE_SOURCE_DIR}=)
+add_link_options(-fdebug-prefix-map=${CMAKE_BINARY_DIR}=)
+if(CMAKE_CXX_COMPILER_VERSION VERSION_GREATER_EQUAL "8")
+    add_compile_options(-ffile-prefix-map=${CMAKE_SOURCE_DIR}=)
+endif()
+
+# TODO: get date from the last git commit to set as epoch
+set(ENV{SOURCE_DATE_EPOCH} 0)
+
 
 #
 # Target configuration
@@ -255,6 +273,7 @@ if(CMAKE_CROSSCOMPILING)
 
     # Create this target before we apply the GC options
     add_library(avr_core STATIC ${AVR_SOURCES})
+    set_reproducible_target(avr_core)
     target_include_directories(avr_core PRIVATE
         ${PRUSA_BOARDS_DIR}/cores/prusa_einsy_rambo/
         ${PRUSA_BOARDS_DIR}/variants/prusa_einsy_rambo/
@@ -319,6 +338,8 @@ function(add_base_binary variant_name)
 endfunction()
 
 function(fw_add_variant variant_name)
+	# Set FW_SOURCES to be reproducible in this variant as it's set in a separate project
+	set_reproducible_sources(FW_SOURCES "Firmware/")
 
 	# Create the Configuration_Prusa.h for this variant so it can be #included.
 	set(VARIANT_CFG_DIR "${CMAKE_CURRENT_BINARY_DIR}/include")

Firmware/Marlin_main.cpp

@@ -1217,8 +1217,6 @@ void setup()
 	SERIAL_ECHOPGM(STRING_VERSION_CONFIG_H);
 	SERIAL_ECHORPGM(_n(" | Author: "));////MSG_AUTHOR
 	SERIAL_ECHOLNPGM(STRING_CONFIG_H_AUTHOR);
-	SERIAL_ECHOPGM("Compiled: ");
-	SERIAL_ECHOLNPGM(__DATE__);
 #endif
 #endif