alsa-lib: conditionally enable libdl in AM_PATH_ALSA m4 macro

The AM_PATH_ALSA macro in utils/alsa.m4 unconditionally uses -ldl. This
breaks compilation of alsa-utils (and probably other packages using this
macro) for targets that do not support dynamic loading, such as for
Blackfin FLAT binaries.

This patch updates the macro to check if dlopen is available, and use that
result to conditionally add -ldl to the list of libraries.

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>

---

diff --git a/utils/alsa.m4 b/utils/alsa.m4
--- a/utils/alsa.m4
+++ b/utils/alsa.m4
@@ -44,6 +44,8 @@ if test "$alsa_inc_prefix" != "" ; then
 fi
 AC_MSG_RESULT($ALSA_CFLAGS)
 
+AC_CHECK_LIB(c, dlopen, LIBDL="", [AC_CHECK_LIB(dl, dlopen, LIBDL="-ldl")])
+
 dnl add any special lib dirs
 AC_MSG_CHECKING(for ALSA LDFLAGS)
 if test "$alsa_prefix" != "" ; then
@@ -52,7 +54,7 @@ if test "$alsa_prefix" != "" ; then
 fi
 
 dnl add the alsa library
-ALSA_LIBS="$ALSA_LIBS -lasound -lm -ldl -lpthread"
+ALSA_LIBS="$ALSA_LIBS -lasound -lm $LIBDL -lpthread"
 LIBS="$ALSA_LIBS $LIBS"
 AC_MSG_RESULT($ALSA_LIBS)