autoconf: Add support for code coverage

Add support for code coverage collection. This helps
developers to check if their test scenarios really exercised
all the OpenOCD functionality that they intended to test.

- Option --enable-gcov has been added to configure.ac
  which enables the coverage collection using Gcov. (Disabled
  by default.)

- The steps to collect and inspect the coverage have been
  described in HACKING file.

Change-Id: I259e401937a255e7ad7f155359a0b7787e4d0752
Signed-off-by: Jan Matyas <jan.matyas@codasip.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/8521
Tested-by: jenkins
Reviewed-by: Evgeniy Naydanov <evgeniy.naydanov@syntacore.com>
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>

Makefile.am

@@ -38,6 +38,7 @@ endif
 
 # common flags used in openocd build
 AM_CFLAGS = $(GCC_WARNINGS)
+AM_LDFLAGS =
 
 AM_CPPFLAGS = $(HOST_CPPFLAGS)\
 			  -I$(top_srcdir)/src \
@@ -51,6 +52,12 @@ AM_CPPFLAGS += -I$(top_srcdir)/jimtcl \
 else
 AM_CPPFLAGS += $(JIMTCL_CFLAGS)
 endif
+
+if USE_GCOV
+AM_CFLAGS += --coverage
+AM_LDFLAGS += --coverage
+endif
+
 EXTRA_DIST += \
 	BUGS \
 	HACKING \