VPATH buils fail with v3.5

[jgraciahlrs]

Building mpiP in-tree works fine. But out-of-tree / VPATH builds fail.

To reproduce:

$> # extract tarball and cd into mpiP directory
$> mkdir mybuild
$> cd mybuild
$> ../configure 
$> make
../arch/arch.h:17:10: fatal error: arch/arch_x86_64.h: No such file or directory

It looks like the include path does not contain $(srcdir).

In fact, calling make as

$> make CPATH=$CPATH:../

serves as a workaround.