Heck, if you really wanted to push the boat out (and I think this might be over the top, but cool) make the lines that fail the compiler directive one faint colour, and the current compiler directive to be shown in normal colours, sorta like, on Linux:
CompilerIf #PB_Compiler_OS = #PB_OS_Linux
ImportC "/usr/lib/libX11.a" + Chr('"') + " -l" + Chr('"') + "GL"
CompilerElse
Import "Opengl32.lib"
CompilerEndIf
; insert hundreds of procedure names
EndImport