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.

.gitignore

@@ -18,6 +18,8 @@ gtk-doc.make
 *.lo
 *.la
 *.loT
+*.gcno
+*.gcda
 libtool
 .deps
 .libs
@@ -165,3 +167,6 @@ plugins/test-service-*
 
 TAGS
 ABOUT-NLS
+
+ModemManager-*-coverage.info
+ModemManager-*-coverage/