clang does not find libbacktrace any more (regression)

[Jason Gunthorpe]

This small example does not compile in Xenial:

#include <backtrace-supported.h>
#if !(BACKTRACE_SUPPORTED)
#error "Not supported"
#endif
#include <backtrace.h>
int main(int argc,const char *argv[])
{
    struct backtrace_state *state = backtrace_create_state(nullptr,false,nullptr,nullptr);

}

$ clang++-3.8 --std=c++14 t.cc -lbacktrace -v
#include <...> search starts here:
 /usr/bin/../lib/gcc/x86_64-linux-gnu/5.3.1/../../../../include/c++/5.3.1
 /usr/bin/../lib/gcc/x86_64-linux-gnu/5.3.1/../../../../include/x86_64-linux-gnu/c++/5.3.1
 /usr/bin/../lib/gcc/x86_64-linux-gnu/5.3.1/../../../../include/c++/5.3.1/backward
 /usr/local/include
 /usr/lib/llvm-3.8/bin/../lib/clang/3.8.0/include
 /usr/include/x86_64-linux-gnu
 /usr/include
End of search list.
t.cc:1:10: fatal error: 'backtrace-supported.h' file not found

While using g++-5 works just fine.

The same example compiles fine on Vivid:

$ clang++-3.6 --std=c++14 t.cc -lbacktrace -v
#include <...> search starts here:
 /usr/bin/../lib/gcc/x86_64-linux-gnu/4.9/../../../../include/c++/4.9
 /usr/bin/../lib/gcc/x86_64-linux-gnu/4.9/../../../../include/x86_64-linux-gnu/c++/4.9
 /usr/bin/../lib/gcc/x86_64-linux-gnu/4.9/../../../../include/c++/4.9/backward
 /usr/local/include
 /usr/lib/llvm-3.6/bin/../lib/clang/3.6.0/include
 /usr/bin/../lib/gcc/x86_64-linux-gnu/4.9/include
 /usr/include/x86_64-linux-gnu
 /usr/include

For some reason the clang install in Xenial does not search the libgcc directory /usr/bin/../lib/gcc/x86_64-linux-gnu/5/include