Aggressively optimize the checks against Boost libraries.

Aggressively optimize the checks against Boost libraries.

	Checking for Boost libraries that require linking was very slow
	because it took several iteration to discover the proper link flags
	to use to properly compile+link a C++ test file.  The test file was
	thus compiled over and over again until the right set of flag is
	found.  Compilation is time-consuming, especially with Boost, so the
	trick is to compile the file once and then try to link it.
	Unfortunately, Autoconf doesn't seem to allow this and always wants
	to delete the intermediate object file.  This file thus bypasses one
	of the Autoconf internal macro to retain the object file.
	On a test case which checks for 2 libraries, the configure time is
	reduced by a factor of 5 on my laptop (~100s -> ~20s).
	* build-aux/boost.m4 (BOOST_FIND_LIB): Compile the test file once
	and then try to link it.
	(_BOOST_AC_LINK_IFELSE): Fork of the Autoconf macro _AC_LINK_IFELSE.
	Optimization suggested by David Herring.

Signed-off-by: Benoit Sigoure <tsuna@lrde.epita.fr>