Enables layouting unwrapped lines around preprocessor directives.

Previously, we'd always start at indent level 0 after a preprocessor
directive, now we layout the following snippet (column limit 69) as
follows:

functionCallTo(someOtherFunction(
    withSomeParameters, whichInSequence,
    areLongerThanALine(andAnotherCall,
  B
                       withMoreParamters,
                       whichStronglyInfluenceTheLayout),
    andMoreParameters),
               trailing);

Note that the different jumping indent is a different issue that will be
addressed separately.

This is the first step towards handling #ifdef->#else->#endif chains
correctly.
Configure: if we compile with clang, check that it is not broken

Some linux distibutions (for example, Mageia 2, Fedora 17) ship Clang that is
essentially broken for the end user.  Clang can not find or compile libstdc++
headers.

The issue is that our configure prefers clang over gcc, thus selecting a broken
Clang when a working GCC is available.

Now we detect this issue by compiling a simple program.  If it does not
compile, configure stops with an error suggesting the user to select a
different compiler.