cmake: use --definitions for gersemi

As pointed out by Divergentdave we can avoid using
`--no-warn-about-unknown-commands` by adding a simple decl file with our
two custom functions.

Author: cpu

cmake/custom_function_defs.txt

@@ -0,0 +1,5 @@
+function(test_binary target_name)
+endfunction()
+
+function(add_c_format_targets targets)
+endfunction()

cmake/format.cmake

@@ -17,15 +17,15 @@ add_custom_target(
 add_custom_target(
     cmake-format-fix
     COMMAND
-        gersemi --no-warn-about-unknown-commands -i CMakeLists.txt
+        gersemi --definitions cmake/custom_function_defs.txt -i CMakeLists.txt
         tests/CMakeLists.txt cmake/*.cmake
     WORKING_DIRECTORY "${CMAKE_CURRENT_SOURCE_DIR}"
 )
 
 add_custom_target(
     cmake-format-check
     COMMAND
-        gersemi --no-warn-about-unknown-commands -c CMakeLists.txt
+        gersemi --definitions cmake/custom_function_defs.txt -c CMakeLists.txt
         tests/CMakeLists.txt cmake/*.cmake
     WORKING_DIRECTORY "${CMAKE_CURRENT_SOURCE_DIR}"
 )