Doxygen: Prevent clang memory leak by adding back the flag reinplaced away by r101062

[cooljeanius (Eric Gallager)]

Removing -Wno-invalid-source-encoding causes clang++ to memory leak when it hits certain files used when compiling doxygen, which makes it take forever, to the point where I usually just end up ^{C-ing it. I can see why removing this flag might be necessary for gcc, but couldn't this reinplace at least be done conditionally? i.e. do the reinplace iff configure.compiler == gcc; on the other hand, if configure.compiler == clang, then don't do it.}

(r101062 for reference)