boost.m4: stylistic issues

* build-aux/boost.m4 (_BOOST_FIND_COMPILER_TAG): Address a couple
of identation issues.

Signed-off-by: Benoit Sigoure <tsunanet@gmail.com>
boost.m4: recognize gcc 4.9 and 4.10

Serial 23.

* build-aux/boost.m4 (_BOOST_FIND_COMPILER_TAG): here.

Signed-off-by: Benoit Sigoure <tsunanet@gmail.com>