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

redoconf/rc/shlib.rc.od

@@ -1,21 +1,43 @@
 . ./redoconf.rc
 rc_include rc/CC.rc
 
-appendln CFLAGS "-fPIC"
-appendln LDFLAGS "-shared"
-
 prog='
 #include <stdlib.h>
 void f() { atoi(""); }
 '
 
+try_unix_style() {
+	(
+		appendln CFLAGS "-fPIC"
+		appendln LDFLAGS "-shared"
+		appendln LDFLAGS "-Wl,-soname,x.so"
+		RCC_NO_MAIN=1 rc_compile cc link "$prog"
+	)
+}
+
+try_macos_style() {
+	(
+		appendln CFLAGS "-fPIC"
+		appendln LDFLAGS "-dynamiclib"
+		appendln LDFLAGS "-current_version"
+		appendln LDFLAGS "1.0"
+		RCC_NO_MAIN=1 rc_compile cc link "$prog"
+	)
+}
+
 if [ -n "$STATIC" ]; then
 	echo "--enable-static specified; not building shared libraries." >&2
 	rc_replaceln HAVE_SHLIB ""
-elif RCC_NO_MAIN=1 rc_compile cc link "$prog"; then
-	rc_replaceln HAVE_SHLIB 1
+elif [ -n "$NOSHARED" ]; then
+	echo "--disable-shared specified; not building shared libraries." >&2
+	rc_replaceln HAVE_SHLIB ""
+elif try_unix_style; then
+	rc_replaceln HAVE_SHLIB UNIX
+elif try_macos_style; then
+	rc_replaceln HAVE_SHLIB MACOS
 else
 	echo "Not building shared libraries on this platform." >&2
+	rc_undo
 	rc_replaceln HAVE_SHLIB ""
 fi
 rc_save