clang-pch actually uses PTH

[erichkeane]

Clang has PreCompiledHeaders and PreTokenizedHeaders, however in PCH mode, boost build uses PTH.

PTH is an un-documented(from a user perspective) failed experimental feature that has a pretty severe error in it that will result in Boost failing in PCH mode on clang. As a result (and since Build seems to be the only user of PTH), I've been encouraged on the clang compiler to remove PTH support, which I'm in the process of doing now.

I suggest that the Boost Build project switch to true PCH if possible.

References to PTH (both include and emit): https://github.com/boostorg/build/blob/develop/src/tools/clang-linux.jam#L105 https://github.com/boostorg/build/blob/develop/src/tools/clang-linux.jam#L128 https://github.com/boostorg/build/blob/develop/src/tools/clang-linux.jam#L140 https://github.com/boostorg/build/blob/develop/src/tools/clang-linux.jam#L148

I also note that the file seems to refer to PTH and PCH interchangeably (99 calls it without-ptc and line 103 calls it 'with-pch', so that perhaps could be cleaned up as well.

[stale[bot]]

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