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.

src/Makefile.am

@@ -8,6 +8,9 @@ udevrules_DATA = \
 	77-mm-usb-serial-adapters-greylist.rules \
 	80-mm-candidate.rules
 
+AM_CFLAGS = $(CODE_COVERAGE_CFLAGS)
+AM_LDFLAGS = $(CODE_COVERAGE_LDFLAGS)
+
 noinst_LTLIBRARIES = libmodem-helpers.la libport.la
 
 libmodem_helpers_la_CPPFLAGS = \

src/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-modem-helpers \
 	test-charsets \