CompilerSettings: Add a separate block for clang definitions.

Sharing compiler settings between GCC and clang does not always work: there
are flags (such as "-fno-check-new" or "-fno-reorder-blocks") that are
specific to GCC, and nothing stops these incompatibilities from becoming
bigger in the future.

Conversely, a separate clang block allows us to pass some additional flags
to clang that would have required yet another if() in the GCC block. For
now, this amounts to "-fdelayed-template-parsing".

(For KDE4, we also need -Wno-return-type-c-linkage because kdepim's
ktexteditorkabcbridge.cpp exports a function that returns a QString with C
linkage, but I hope this can be solved in a different way for kdepim5).

Last but not least, checks for bad GCC allocators or support for some flags
which are always present in clang can be avoided altogether when we know the
compiler we are using.

REVIEW: 112136