configure: no need to run GMP version check against compile-time constants -> cross-compile friendly.
Moved to more standard (read: autotools) set of make targets.

The following was accomplished mostly by using the '$target-local' rules instead of '$target' or '$target-am'.
* 'make distclean' won't remove ./configure or any tools it needs anymore.
* 'make maintainer-clean' removes any left-overs from configure.
* 'make all' or just 'make' won't call the 'check' target anymore (but there is a textual hint to the user now as in e.g. GMP).
First step to build out of tree - in-tree doesn't work anymore, though (symlink to itself).
Moved obj/MPFR to src/MPFR + reworked configure's and compile-time backend selection.

Details:
 * out-of-tree as well as in-tree builds work now.
 * config.status now takes care about creating symlinks (no more "already exists" errors)
 * configure option '--with-backend' is now called '--with-backends' to allow for more than one.
   It takes a comma-separated list of available backends; MPFR is the default.
 * introduced macro iRRAM_BACKENDS (string containing the inbuilt backend names)
 * introduced macro iRRAM_BACKEND_MPFR (#define'd to 0 or 1) and moved conditional includes to use it

The above macros are #define'd in include/iRRAM/version.h as it was easiest to expand since the file is
already processed by configure. Maybe we need another place for these macros like 'config.h'...
Fix for out-of-tree build + renamed variable INCLUDES to AM_CPPFLAGS to silence autoconf.
CPPFLAGS_INST needs to include user-defined include_path from configure time to be able to find e.g. <gmp.h>.