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

4 files changed, 26 insertions, 14 deletions

diff --git a/.travis.yml b/.travis.yml
index 7ceaddf..2dbb498 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -22,5 +22,6 @@ addons:
 script:
   - clang-format-3.8 -i $(find . -name "*.[ch]" | tr '\n' ' ') && git diff --exit-code || (echo 'Code was not formatted using clang-format!'; false)
   # TODO: re-enable sanitizers once issues are fixed
-  - autoreconf -fi && mkdir -p build && cd build && (../configure --disable-sanitizers || (cat config.log; false)) && make -j V=1
-  - make -j check V=1 || (cat test-suite.log; false)
+  - autoreconf -fi && mkdir -p build && cd build && (../configure --disable-sanitizers || (cat config.log; false)) && make -j 8 V=1 && ldd ./i3status |& grep -q pulse || (echo "not linked against pulseaudio"; exit 1)
+  - make clean && (../configure --disable-sanitizers --disable-pulseaudio || (cat config.log; false)) && make -j 8 V=1 && ldd ./i3status |& grep -q pulse && (echo "linked against pulseaudio"; exit 1) || true
+  - make -j 8 check V=1 || (cat test-suite.log; false)
diff --git a/configure.ac b/configure.ac
index a6c31d7..11caa33 100644
--- a/configure.ac
+++ b/configure.ac
@@ -80,25 +80,29 @@ AC_CANONICAL_HOST
 PKG_CHECK_MODULES([CONFUSE], [libconfuse])
 PKG_CHECK_MODULES([YAJL], [yajl])
 
-pulse=true
+AC_ARG_ENABLE(pulseaudio,
+  AS_HELP_STRING(
+    [--disable-pulseaudio],
+    [build without pulseaudio support]),
+  [ax_pulse=$enableval],
+  [ax_pulse=yes])
+AM_CONDITIONAL([PULSE], [test x$ax_pulse = xyes])
+AS_IF([test x"$ax_pulse" = x"yes"],
+      [PKG_CHECK_MODULES([PULSE], [libpulse])])
+pulse_def=0
+AS_IF([test x"$ax_pulse" = x"yes"],
+      [pulse_def=1])
+AC_DEFINE_UNQUOTED([HAS_PULSEAUDIO], [$pulse_def], [Build with pulseaudio])
+
 case $host_os in
 	linux*)
 	PKG_CHECK_MODULES([NLGENL], [libnl-genl-3.0])
 	PKG_CHECK_MODULES([ALSA], [alsa])
 	;;
-	openbsd*)
-	pulse=false
-	;;
-	dragonfly*)
-	pulse=false
-	;;
 	netbsd*)
 	AC_SEARCH_LIBS([prop_string_create], [prop])
 	;;
 esac
-AM_CONDITIONAL([PULSE], [test x$pulse = xtrue])
-AS_IF([test x"$pulse" = x"true"],
-      [PKG_CHECK_MODULES([PULSE], [libpulse])])
 
 dnl TODO: check for libbsd for GNU/kFreeBSD
 
@@ -160,6 +164,7 @@ AS_HELP_STRING([is release version:], [${is_release}])
 AS_HELP_STRING([enable debug flags:], [${ax_enable_debug}])
 AS_HELP_STRING([code coverage:], [${CODE_COVERAGE_ENABLED}])
 AS_HELP_STRING([enabled sanitizers:], [${ax_enabled_sanitizers}])
+AS_HELP_STRING([pulseaudio support:], [${ax_pulse}])
 
 To compile, run:
 
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:
diff --git a/src/print_volume.c b/src/print_volume.c
index 91e8ce2..7364d47 100644
--- a/src/print_volume.c
+++ b/src/print_volume.c
@@ -86,7 +86,7 @@ void print_volume(yajl_gen json_gen, char *buffer, const char *fmt, const char *
         free(instance);
     }
 
-#if !defined(__DragonFly__) && !defined(__OpenBSD__)
+#if HAS_PULSEAUDIO
     /* Try PulseAudio first */
 
     /* If the device name has the format "pulse[:N]" where N is the