build-sys: integrate coverage report

Just pass --enable-coverage to configure and then "make coverage" to
have the coverage report at coverage dir.
diff --git a/.gitignore b/.gitignore
index 88c8149..f60cac6 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,4 +1,6 @@
 *.o
+*.gcda
+*.gcno
 /*.tar.xz
 /*.md5sum
 .deps/
@@ -14,6 +16,7 @@
 /config.log
 /config.status
 /configure
+/coverage
 /libtool
 /stamp-h1
 /test-suite.log