Fix some build problems on MacOS X.

- Linking shared libraries needs slightly different options.

- We were trying to detect mach_time.h but needed to detect
  mach/mach_time.h instead.

While we're here, add a --disable-shared option to ./configure, which
is different from --enable-static.  --disable-shared does not build
*new* share libraries, but doesn't pass -static to the linker
(apparently there is no static linking posible on MacOS).

redoconf/rc/default.h.rc.od

@@ -2,8 +2,9 @@
 rc_include rc/CC.rc
 
 base="${1#*/}"
-h="${base%.rc}"
-H=$(echo "$h" | tr 'a-z.' 'A-Z_')
+h1="${base%.rc}"
+h=$(echo "$h1" | sed -e 's,__,/,g')  # x__y_z.h.rc.od -> <x/y_z.h>
+H=$(echo "$h1" | tr 'a-z.' 'A-Z_')
 
 if rc_compile cc nolink "#include <$h>"; then
 	rc_replaceln "HAVE_$H" 1