make pulseaudio an optional dependency, follow best practices

For my thoughts about optional dependencies, see
https://michael.stapelberg.ch/posts/2019-05-23-optional-dependencies/

This commit follows the best practices outlined in that article:

1. The travis config was modified to verify both code paths build and link/don’t
   link against pulseaudio.

2. If pulseaudio is missing, the build fails until packagers explicitly pass a
   --disable flag. In practice, I think the only situation when this flag should
   be set is in source-based linux distributions where users can express
   package-level compilation preferences (e.g. Gentoo USE flags).

3. The --version output now reflects the status of the optional dependency.

fixes #359

1 files changed, 7 insertions, 1 deletions

diff --git a/i3status.c b/i3status.c
index 0898da3..1ab8400 100644
--- a/i3status.c
+++ b/i3status.c
@@ -565,7 +565,13 @@ int main(int argc, char *argv[]) {
                 return 0;
                 break;
             case 'v':
-                printf("i3status " VERSION " © 2008 Michael Stapelberg and contributors\n");
+                printf("i3status " VERSION " © 2008 Michael Stapelberg and contributors\n"
+#if HAS_PULSEAUDIO
+                       "Built with pulseaudio support\n"
+#else
+                       "Built without pulseaudio support\n"
+#endif
+                       );
                 return 0;
                 break;
             case 0: