build: add code coverage support

Build all targets, except for CLI tools (mmcli, uml290), with special
flags needed for collecting code coverage information when the build has
been configured with --enable-code-coverage.

Three new targets are available in the top build directory:

 - `check-code-coverage' runs the test suite and generates a code
   coverage report,

 - `code-coverage-capture' generates a code coverage report from already
   collected data, which can come in handy when one wants to see code
   paths touched by a particular test,

 - `code-coverage-clean' removes the collected coverage data and the
   generated reports.

libqcdm/src/Makefile.am

@@ -1,5 +1,7 @@
-noinst_LTLIBRARIES = libqcdm.la libqcdm-test.la
+AM_CFLAGS = $(CODE_COVERAGE_CFLAGS)
+AM_LDFLAGS = $(CODE_COVERAGE_LDFLAGS)
 
+noinst_LTLIBRARIES = libqcdm.la libqcdm-test.la
 
 libqcdm_la_CPPFLAGS = \
 	$(MM_CFLAGS)

libqcdm/tests/Makefile.am

@@ -1,5 +1,8 @@
 include $(top_srcdir)/gtester.make
 
+AM_CFLAGS = $(CODE_COVERAGE_CFLAGS)
+AM_LDFLAGS = $(CODE_COVERAGE_LDFLAGS)
+
 noinst_PROGRAMS = test-qcdm modepref ipv6pref reset
 TEST_PROGS += test-qcdm