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/configure.sh

@@ -50,7 +50,8 @@ usage() {
 
     --prefix=       Change installation prefix (usually /usr/local)
     --host=         Architecture prefix for output (eg. i686-w64-mingw32-)
-    --enable-static               Link libraries and binaries statically
+    --enable-static               Link binaries statically
+    --disable-shared              Do not build shared libraries
     --{dis,en}able-optimization   Disable/enable optimization for C/C++
     --{dis,en}able-debug          Disable/enable debugging flags for C/C++
     -h, --help      This help message
@@ -115,6 +116,9 @@ for d in "$@"; do
 		--enable-static)
 			emit "STATIC" "1"
 			;;
+		--disable-shared)
+			emit "NOSHARED" "1"
+			;;
 		--enable-optimization)
 			emit_append OPTFLAGS "-O2"
 			;;