Add xgpu.m4 for use by autotools-based clients

This adds two new install directories to Makefile:

datadir ?= $(prefix)/share
aclocaldir ?= $(datadir)/aclocal

The file xgpu.m4 in installed in $(aclocaldir) during "make install".
It can be used in configure.ac by autotools-based clients to provide
a "--with-xgpu" configure option and to check for the existence of xGPU
at the indicated location (defaulting to /use/local).  It can also be
used to run xgpuinfo to set some xGPU sizing parameters in config.h.

Quick-start: Just add "AX_CHECK_XGPUINFO" to your configure.ac file.

The xgup.m4 defines to autoconf macros:

* AX_CHECK_XGPU

This macro adds the "--with-xgpu" option to configure and also checks
whether xGPU headers and libraries can be found at the specified
location.  First is checks in DIR/include and DIR/lib.   If that fails,
it falls back to checking in just DIR.  If that fails, configure fails.
This macro defines to AC_SUBST values that can be used in Makefile.am:
XGPU_INCDIR and XGPU_LIBDIR.

* AX_CHECK_XGPUINFO

This macro call AX_CHECK_XGPU, then checks for and uses the xgpuinfo
utility to derive sizing info for the xGPU installation.  It defines the
following macros in config.h for the xGPU sizing info:

XGPU_NSTATION   - Number of dual-pol(!) stations per xGPU instance
XGPU_NFREQUENCY - Number of frequency channels per xGPU instance
XGPU_NTIME      - Number of time samples per freqency channel per xGPU
                  instance
Make uninstall target more thorough

Now it also uninstalls cuda_correlator and xgpu.m4.