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>

HACKING

@@ -92,6 +92,22 @@ patch:
   make
   @endcode
 
+- Code coverage analysis
+
+  By inspecting the code coverage, you can identify potential gaps in your testing
+  and use that information to improve your test scenarios.
+
+  Example usage:
+  @code
+  mkdir build-gcov; cd build-gcov
+  ../configure --enable-gcov [...]
+  make
+  # ... Now execute your test scenarios to collect OpenOCD code coverage ...
+  lcov --capture --directory ./src --output-file openocd-coverage.info
+  genhtml openocd-coverage.info --output-directory coverage_report
+  # ... Open coverage_report/index.html in a web browser ...
+  @endcode
+
 Please consider performing these additional checks where appropriate
 (especially Clang Static Analyzer for big portions of new code) and
 mention the results (e.g. "Valgrind-clean, no new Clang analyzer