build: Add a code coverage option

Configure with:
  ./configure --enable-code-coverage
Show coverage output with:
  make code-coverage-capture

Built on m4/ax_code_coverage.m4 from the GNU autoconf archive
to provide the macros to check for presence of gcov + lcov;
upstream macro modified to:
 - Remove use of $(AM_DEFAULT_VERBOSITY) , as earlier versions of
   autoconf (such as the one used by default on Travis) do not have this.
 - Rather than automatically defining CODE_COVERAGE_RULES to be a set
   of makefile rules that use ifeq/endif (which is GNU make-specific),
   instead only define CODE_COVERAGE_RULES if coverages is turned on,
   and in that case don't use conditionals in the makefile.

.gitignore

@@ -24,12 +24,18 @@ ltmain.sh
 compile
 libcares.pc
 missing
+test-driver
 c-ares-*.tar.gz
 INSTALL
 install-sh
 *.o
 *.lo
 *.la
+*.gcno
+*.gcda
+*.gcov
+c-ares---coverage.info
+c-ares---coverage
 mkinstalldirs
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