Misc: Simplify Haiku build and fix broken parallel build

Misc: Simplify Haiku build and fix broken parallel build

The Haiku build was previously being done as a nested package
due to its C++ requirement, but for some reason setting SUBDIR
in an automake conditional breaks parallel builds. To fix this
and simplify the Haiku build process, this commit adds an
unconditional check for a C++ compiler using AC_PROG_CXX and
builds the Haiku sources as part of the main libusb project.

Note that AC_PROG_CXX itself does not cause the configure script
to fail if a C++ compiler is not found. Therefore on non-Haiku
platforms there is no requirement to have a C++ compiler installed
in order to build libusb.

Closes #121

Signed-off-by: Chris Dickens <christopher.a.dickens@gmail.com>