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/link-shlib.od

@@ -2,7 +2,7 @@
 rc_include _all.rc rc/shlib.rc
 
 # Tricky quoting: see _compile.od for details.
-if [ "$HAVE_SHLIB" = 1 ]; then
+if [ "$HAVE_SHLIB" = UNIX ]; then
 	cat >$3 <<-EOF
 		#!/bin/sh -e
 		LINK=$(shquote "$LINK")
@@ -20,6 +20,25 @@ if [ "$HAVE_SHLIB" = 1 ]; then
 		  "\$@" \\
 		  \$LIBS
 	EOF
+elif [ "$HAVE_SHLIB" = MACOS ]; then
+	cat >$3 <<-EOF
+		#!/bin/sh -e
+		LINK=$(shquote "$LINK")
+		LDFLAGS=$(shquote "$LDFLAGS")
+		OPTFLAGS=$(shquote "$OPTFLAGS")
+		LIBS=$(shquote "$LIBS")
+		LIBDIR=$(shquote "$LIBDIR")
+		o="\$1"
+		ob="\${o#*/}"
+		shift
+		IFS="$NL"
+		set -f
+		\$LINK -dynamiclib -o "\$o" \\
+		  -install_name "\$ob" \\
+		  \$LDFLAGS \$OPTFLAGS \\
+		  "\$@" \\
+		  \$LIBS
+	EOF
 else
 	# If no shared library support and we try to build one,
 	# compensate by building a static library instead in the