CMakeColors.cmake: Use colors only if stderr is an interactive terminal.

In particular, do not write ugly escape sequences to log files.

Fixes #124.

CMakeModules/CMakeColors.cmake

@@ -1,4 +1,8 @@
 if(NOT WIN32)
+  # [ -t 2 ] tests whether stderr is interactive.
+  # The negation '!' is because for POSIX shells, 0 is true and 1 is false.
+  execute_process(COMMAND [ ! -t 2 ] RESULT_VARIABLE IS_STDERR_INTERACTIVE)
+  if(IS_STDERR_INTERACTIVE)
     string(ASCII 27 Esc)
     set(ColorReset  "${Esc}[m")
     set(ColorBold   "${Esc}[1m")
@@ -16,4 +20,5 @@ if(NOT WIN32)
     set(BoldMagenta "${Esc}[1;35m")
     set(BoldCyan    "${Esc}[1;36m")
     set(BoldWhite   "${Esc}[1;37m")
+  endif(IS_STDERR_INTERACTIVE)
 endif()