build-sys: make sure LD_AS_NEEDED=1 does not break AC_LINK_IFELSE.

build-sys: make sure LD_AS_NEEDED=1 does not break AC_LINK_IFELSE.

For some combinations of autoconf/gcc (e.g. 2.69/4.7.2) setting
LD_AS_NEEDED to 1 breaks AC_LINK_IFELSE. That macro generates
a compilation command line where the libraries precede the generated
C source code. With LD_AS_NEEDED=1 all referenced/tested symbols
from any of the libraries end up being undefined. This fools
AC_LINK_IFELSE to consider the test a failure and always choose the
else branch. rpmbuild always sets LD_AS_NEEDED to 1. To work around
this save, unset and restore LD_AS_NEEDED for the duration of the
AC_LINK_IFELSE tests.