configure: Fix restoring LDFLAGS.

configure: Fix restoring LDFLAGS.

The configure script saves the values of CPPFLAGS, CFLAGS and LDLFAGS
before each check and restores the old values afterwards. However,
for LDLFAGS this was broken at several places, since we used LD_FLAGS
(with an underscore) for saving the value and LDFLAGS for restoring it
(or vice versa).

This bug had the effect that a user-specified value of LDFLAGS was
ignored in some tests because LDFLAGS was restored to the empty
value LD_FLAGS during an earlier test.

This patch changes all occurrences of LD_FLAGS in configure.ac to
LDFLAGS.