introduce define for FST tracing (#2592)

This is to allow C++ verilator toplevel to support
multiple modes of waveform tracing
VM_TRACE_FST can be used inside a #if VM_TRACE
section to switch between classic .vcd tracing and the
more compact .fst format supported by GTKWAVE

docs/CONTRIBUTORS

@@ -40,6 +40,7 @@ Lukasz Dalek
 Maarten De Braekeleer
 Maciej Sobkowski
 Marco Widmer
+Markus Krause
 Marshal Qiao
 Matthew Ballance
 Michael Killough

include/verilated.mk.in

@@ -54,6 +54,7 @@ VK_CPPFLAGS_ALWAYS += \
 		-DVM_COVERAGE=$(VM_COVERAGE) \
 		-DVM_SC=$(VM_SC) \
 		-DVM_TRACE=$(VM_TRACE) \
+		-DVM_TRACE_FST=$(VM_TRACE_FST) \
 		$(CFG_CXXFLAGS_NO_UNUSED) \
 
 ifeq ($(CFG_WITH_CCWARN),yes)	# Local... Else don't burden users

src/V3EmitMk.cpp

@@ -65,6 +65,10 @@ public:
         of.puts("VM_TRACE = ");
         of.puts(v3Global.opt.trace() ? "1" : "0");
         of.puts("\n");
+        of.puts("# Tracing output mode in FST format?  0/1 (from --trace-fst)\n");
+        of.puts("VM_TRACE_FST = ");
+        of.puts(v3Global.opt.trace() && v3Global.opt.traceFormat().fst() ? "1" : "0");
+        of.puts("\n");
         of.puts("# Tracing threaded output mode?  0/1/N threads (from --trace-thread)\n");
         of.puts("VM_TRACE_THREADS = ");
         of.puts(cvtToStr(v3Global.opt.trueTraceThreads()));