Document the flag needed for function names in stack backtraces.

Glibc and Apple's libc both provide support for stack introspection.
This is used by the invariant code to generate a stack backtrace
on invariant failure.  By default GCC strips out the function names
so an additional LINKFLAG is needed to get function names in the
backtrace.  This was hidden^Wdocumented in util/invariant.cpp, this
patch adds it to config.inc so it can be more widely understood
and used.

Author: martin

src/config.inc

@@ -12,6 +12,9 @@ endif
 #CXXFLAGS += -O2 -DNDEBUG
 #CXXFLAGS += -O0 -g
 
+# With GCC this adds function names in stack backtraces
+#LINKFLAGS="-rdynamic"
+
 # If GLPK is available; this is used by goto-instrument and musketeer.
 #LIB_GLPK = -lglpk