configure: Fix --enable-XX-bit flags by moving LT_INIT where it should

configure: Fix --enable-XX-bit flags by moving LT_INIT where it should

Moving LT_INIT after setting completely (AM_)C(XX)FLAGS and LDFLAGS.
LT_INIT needs them as they are expected to be used all along
the compilation when the macro runs its tests to determine among other
things the host type.

For info, see http://www.gnu.org/software/libtool/manual/html_node/LT_005fINIT.html

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=50754
Signed-off-by: Alexandre Demers <alexandre.f.demers@gmail.com>
Tested-by: Tapani Palli <lemody@gmail.com>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>