bootstrap: Convert MSYS paths to Windows format (#13001)

We set CMAKE_PREFIX_PATH from the --prefix= option.  The calling shell
might not translate "/c/..." to "c:/..." paths but we need to store
Windows paths in CMake cache variables.  Pass the specified path through
the MSYS shell in a form it will convert to a Windows path using the
MSYS fstab.

Some MSYS bash implementations leave trailing space on the command line
to 'cmd /c echo ...' after quoting the message.  The Windows echo tool
preserves both the quotes and the trailing space.  Use a sed expression
that strips quotes and trailing spaces after the end quote.

bootstrap

@@ -127,10 +127,20 @@ fi
 cmake_bootstrap_dir="${cmake_binary_dir}/Bootstrap${_cmk}"
 
 # Helper function to fix windows paths.
-cmake_fix_slashes ()
-{
+case "${cmake_system}" in
+*MINGW*)
+  cmake_fix_slashes()
+  {
+    cmd //c echo "$(echo "$1" | sed 's/\\/\//g')" | sed 's/^"//;s/" *$//'
+  }
+  ;;
+*)
+  cmake_fix_slashes()
+  {
     echo "$1" | sed 's/\\/\//g'
-}
+  }
+  ;;
+esac
 
 # Choose the default install prefix.
 if ${cmake_system_mingw}; then