Re-enable with-libxmp configure option

Mostly in a standards-compliant way this time, complain if set as "no".
This is being re-added by user request.

Signed-off-by: Claudio Matsuoka <cmatsuoka@gmail.com>

configure.ac

@@ -8,6 +8,8 @@ AC_ARG_ENABLE(alsa,
   [  --disable-alsa          don't compile ALSA support])
 AC_ARG_ENABLE(pulseaudio,
   [  --enable-pulseaudio     compile PulseAudio support])
+AC_ARG_WITH(libxmp,
+  [  --with-libxmp=<path>    libxmp prefix (optional)])
 
 AC_SUBST(LD_VERSCRIPT)
 AC_CANONICAL_HOST
@@ -28,7 +30,21 @@ esac
 AC_PROG_INSTALL
 AM_INIT_AUTOMAKE([foreign subdir-objects tar-pax])
 
-PKG_CHECK_MODULES([libxmp], [libxmp >= 4.1])
+dnl For the case where we don't have pkgconfig and we have the library
+dnl installed in a non-default location
+
+if test "x$with_libxmp" = "xno"; then
+    AC_ERROR(libxmp is required)
+fi
+if test "x$with_libxmp" != "x"; then
+    CPPFLAGS="${CPPFLAGS} -I${with_libxmp}/include"
+    LDFLAGS="${LDFLAGS} -L${with_libxmp}/lib"
+    if test ! -f "${with_libxmp}/include/xmp.h"; then
+        AC_ERROR(can't find ${with_libxmp}/include/xmp.h)
+    fi
+else
+    PKG_CHECK_MODULES([libxmp], [libxmp >= 4.1])
+fi
 
 dnl Don't use things like /usr/etc or /usr/var