fix sys/soundcard.h related build failure on FreeBSD

The FreeBSD build was broken because special typedefs like `u_long`
and `u_char` weren't known when including `sys/soundcard.h` during
compile.

When we introduced some strict POSIX/C11 flags with commit
fad6a3e, this seems to have disabled the __BSD_VISIBLE flag on
FreeBSD which made these typedefs disappear (there is a
`#if __BSD_VISIBLE` guard in `sys/types.h`).

We don't know the reason for disabling `__BSD_VISIBLE` (perhaps it is
along the line of "if you want POSIX, you don't get BSD enhancements")
and we don't know if there are adverse side-effects to manually
enabling `__BSD_VISIBLE`, but for now it works.

If anybody has deeper understanding of this and a better way to fix
this, please speak up :)

This fixes issue #105.

(n.b.: `sys/soundcard.h` contains two sound systems: AIO and OSS.  The
 missing typedefs are only used for AIO while we only need OSS.  So if
 that header file would have been split into two seperate files, we
 would not have run into this problem.  The OSS part should work fine
 with the POSIX/C11 flags without setting `__BSD_VISIBLE`.)

configure

@@ -762,6 +762,39 @@ if [ "$use_devdsp" != no ]; then
     remember_use devdsp
     check_include sys/soundcard.h
     use_devdsp="$have_sys_soundcard_h"
+    # this check is needed for FreeBSD but it only works when the C11 flags are used
+    C11_FLAGS="-D_XOPEN_SOURCE=500 -D_POSIX_C_SOURCE=200809L"
+    if [ "$have_sys_soundcard_h" = yes ]; then
+	if ! cc_check "checking if we need additional flags for sys/soundcard.h" "" "$C11_FLAGS" "no" "yes" <<EOF; then
+#include <sys/soundcard.h>
+int main(int argc, char **argv)
+{
+    #ifdef OPEN_SOUND_SYSTEM
+    return 0;
+    #else
+    return 1;
+    #endif
+}
+EOF
+
+            if cc_check "checking if we need __BSD_VISIBLE for sys/soundcard.h" "" "$C11_FLAGS -D__BSD_VISIBLE" "yes" "no" <<EOF; then
+#include <sys/soundcard.h>
+int main(int argc, char **argv)
+{
+    #ifdef OPEN_SOUND_SYSTEM
+    return 0;
+    #else
+    return 1;
+    #endif
+}
+EOF
+                append_nodupe CFLAGS "-D__BSD_VISIBLE"
+	    else
+		echo "no way found to make sys/soundcard.h work"
+	        use_devdsp=no
+	    fi
+	fi
+    fi
     recheck_use devdsp
 fi