Meta: Print error if cmake does not exist

Author: https://github.com/demostanis
Commit: 1c979889b2
Pull-request: https://github.com/SerenityOS/serenity/pull/15104
Reviewed-by: https://github.com/timschumi

Meta/CMake/processor-count.cmake

@@ -1,3 +1,5 @@
 include(ProcessorCount)
 ProcessorCount(N)
-message("${N}")
+# Executing echo here allows us to print to the standard output,
+# to separate the processor count from potential errors
+execute_process(COMMAND "${CMAKE_COMMAND}" -E echo "${N}")