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>

configure.ac

@@ -171,6 +171,9 @@ m4_define([DUMMY_ADAPTER],
 m4_define([OPTIONAL_LIBRARIES],
 	[[[capstone], [Use Capstone disassembly framework], []]])
 
+m4_define([COVERAGE],
+	[[[gcov], [Collect coverage using gcov], []]])
+
 AC_ARG_ENABLE([doxygen-html],
   AS_HELP_STRING([--disable-doxygen-html],
     [Disable building Doxygen manual as HTML.]),
@@ -199,6 +202,19 @@ AC_ARG_ENABLE([werror],
   AS_HELP_STRING([--disable-werror], [Do not treat warnings as errors]),
   [gcc_werror=$enableval], [gcc_werror=$gcc_warnings])
 
+AC_ARG_ENABLE([gcov],
+  AS_HELP_STRING([--enable-gcov], [Enable runtime coverage collection via gcov]),
+  [enable_gcov=$enableval], [enable_gcov=no])
+
+AS_IF([test "x$enable_gcov" = "xyes"], [
+  AC_DEFINE([USE_GCOV], [1], [1 to enable coverage collection using gcov.])
+  dnl When collecting coverage, disable optimizations.
+  dnl This overrides the "-O2" that autoconf uses by default:
+  CFLAGS+=" -O0"
+], [
+  AC_DEFINE([USE_GCOV], [0], [0 to leave coverage collection disabled.])
+])
+
 # set default verbose options, overridden by following options
 debug_usb_io=no
 debug_usb_comms=no
@@ -787,6 +803,8 @@ AM_CONDITIONAL([INTERNAL_JIMTCL], [test "x$use_internal_jimtcl" = "xyes"])
 AM_CONDITIONAL([HAVE_JIMTCL_PKG_CONFIG], [test "x$have_jimtcl_pkg_config" = "xyes"])
 AM_CONDITIONAL([INTERNAL_LIBJAYLINK], [test "x$use_internal_libjaylink" = "xyes"])
 
+AM_CONDITIONAL([USE_GCOV], [test "x$enable_gcov" = "xyes"])
+
 # Look for environ alternatives.  Possibility #1: is environ in unistd.h or stdlib.h?
 AC_MSG_CHECKING([for environ in unistd.h and stdlib.h])
 AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[
@@ -862,7 +880,8 @@ m4_foreach([adapter], [USB1_ADAPTERS,
 	LIBGPIOD_ADAPTERS,
 	LIBJAYLINK_ADAPTERS, PCIE_ADAPTERS, SERIAL_PORT_ADAPTERS,
 	DUMMY_ADAPTER,
-	OPTIONAL_LIBRARIES],
+	OPTIONAL_LIBRARIES,
+	COVERAGE],
 	[s=m4_format(["%-40s"], ADAPTER_DESC([adapter]))
 	AS_CASE([$ADAPTER_VAR([adapter])],
 		[auto], [