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 \

docs/cookbook/c/monotime.c

@@ -23,7 +23,7 @@ long long monotime(void) {
 	return !result ? 1 : result;
 }
 
-#elif HAVE_MACH_TIME_H
+#elif HAVE_MACH__MACH_TIME_H
 
 #include <mach/mach.h>
 #include <mach/mach_time.h>

docs/cookbook/c/rc/posix.rc.od

@@ -8,13 +8,13 @@ struct timespec x;
 '
 
 x=
-rc_replaceln HAS_POSIX 1
+rc_replaceln HAVE_POSIX 1
 if ! rc_compile cc link "$prog"; then
 	x="-D_XOPEN_SOURCE=500"
 	rc_appendln CPPFLAGS "$x"
 	if ! rc_compile cc link "$prog"; then
 		rc_undo
-		rc_replaceln HAS_POSIX ""
+		rc_replaceln HAVE_POSIX ""
 	fi
 fi
 rc_save