pch-warnings

• The PCH_WARNINGS target property, and its initializing CMAKE_PCH_WARNINGS variable, were added to enable compiler warnings for the headers collected into a precompiled header. Previously the generated precompiled header was always marked as a system header, which suppressed those warnings.

  The system header marker now comes from an internal CMAKE_PCH_SYSTEM_HEADER_PRAGMA variable instead of from CMAKE_PCH_PROLOGUE. Projects that cleared or replaced CMAKE_PCH_PROLOGUE in order to drop the marker no longer affect it, and must set PCH_WARNINGS or CMAKE_PCH_WARNINGS instead.