build: Add ability to generate gprof output

Suitable for profiling. We've had this before 2.0.
As earlier, pie code is not profiled as well as
syscalls library.

So simply build as "make GMON=1".
Signed-off-by: Cyrill Gorcunov <gorcunov@virtuozzo.com>
Reviewed-by: Dmitry Safonov <dsafonov@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>

Makefile

@@ -101,6 +101,12 @@ else
         CFLAGS		+= -O2 -g
 endif
 
+ifeq ($(GMON),1)
+        CFLAGS		+= -pg
+        GMONLDOPT	+= -pg
+export GMON GMONLDOPT
+endif
+
 CFLAGS			+= $(WARNINGS) $(DEFINES)
 
 #