Fix clang-cl dependency builds failing on a precompiled header from the compiler cache

cmake/modules/ClangClShowIncludes.cmake

@@ -1,10 +1,18 @@
 # ccache does not parse the -clang: arguments CMake uses for clang-cl's gcc-style
 # depfile, so a cache hit writes the object and no depfile, and Ninja then records
 # no headers for that object. ccache reproduces /showIncludes output on a hit.
+#
+# Clang also records the modification time of every input in a precompiled header
+# and rejects the header once one differs. ccache caches the compile that creates
+# the header, so a tree that regenerates cmake_pch.cxx gets back a header stamped
+# with the old file's time, and every compile that uses it fails. The build system
+# already rebuilds the header when an input changes. This covers the dependencies,
+# such as wxWidgets; PrecompiledHeader.cmake does the same for OrcaSlicer's targets.
 foreach (_lang C CXX)
     if (CMAKE_${_lang}_COMPILER_ID STREQUAL "Clang" AND
         CMAKE_${_lang}_COMPILER_FRONTEND_VARIANT STREQUAL "MSVC")
         set(CMAKE_DEPFILE_FLAGS_${_lang} "/showIncludes")
         set(CMAKE_${_lang}_DEPFILE_FORMAT msvc)
+        list(APPEND CMAKE_${_lang}_COMPILE_OPTIONS_CREATE_PCH -Xclang -fno-pch-timestamp)
     endif ()
 endforeach ()