Take -DWX_PRECOMP out of GCC_PRAGMA_FLAGS and put it into its own PCH_FLAGS.

Take -DWX_PRECOMP out of GCC_PRAGMA_FLAGS and put it into its own PCH_FLAGS.
If -DNO_GCC_PRAGMA is included in GCC_PRAGMA_FLAGS (because the platform
always requires it) then it won't be duplicated in PCH_FLAGS, but if
the platform does not normally need -DNO_GCC_PRAGMA then the flag
will be included in PCH_FLAGS because PCH always requires it.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@31471 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775