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).

docs/cookbook/c/all.rc.od

@@ -10,7 +10,7 @@ rc_include \
 	rc/rt.autolib.rc \
 	rc/libpng.rc \
 	rc/clock_gettime.rc \
-	rc/mach_time.h.rc \
+	rc/mach__mach_time.h.rc \
 	rc/windows.h.rc \
 	rc/posix.rc \
 	rc/printf_lld.rc \