bootstrap: Do not suppress CMAKE_OSX_SYSROOT if CFLAGS have -isysroot (#14324)

Revert commit a1c032b9 (bootstrap: Suppress CMAKE_OSX_SYSROOT if CFLAGS
have -isysroot, 2012-09-21).  If MACOSX_DEPLOYMENT_TARGET is set then
CMAKE_OSX_DEPLOYMENT_TARGET will be set and Darwin.cmake will complain
if no CMAKE_OSX_SYSROOT is set.  Just allow both -isysroot flags to
appear.  The one generated by CMAKE_OSX_SYSROOT appears after and
overrides the one from CFLAGS/CXXFLAGS.

bootstrap

@@ -1525,14 +1525,6 @@ 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 '