bootstrap: Suppress CMAKE_OSX_SYSROOT if CFLAGS have -isysroot

In order to bootstrap on OS X with Xcode without the command line tools
one must add -isysroot to CFLAGS and CXXFLAGS.  In this case the flags
will make it into the configured CMake.  Set CMAKE_OSX_SYSROOT to empty
in the initial cache to prevent CMake from adding -isysroot again.

bootstrap

@@ -1505,6 +1505,14 @@ set (CMAKE_MAN_DIR "'"${cmake_man_dir}"'" CACHE PATH "Install location for man p
 set (CMAKE_DATA_DIR "'"${cmake_data_dir}"'" CACHE PATH "Install location for data (relative to prefix)." FORCE)
 ' > "${cmake_bootstrap_dir}/InitialCacheFlags.cmake"
 
+# Suppress -isysroot if user-provided flags already have it.
+if echo "${cmake_c_flags}"   | grep isysroot >/dev/null 2>&1 &&
+   echo "${cmake_cxx_flags}" | grep isysroot >/dev/null 2>&1; then
+  echo '
+set(CMAKE_OSX_SYSROOT "" CACHE PATH "" FORCE)
+' >> "${cmake_bootstrap_dir}/InitialCacheFlags.cmake"
+fi
+
 # Add configuration settings given as command-line options.
 if [ "x${cmake_bootstrap_qt_gui}" != "x" ]; then
   echo '