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If the --with-ldopts option is not passed on the command line, respect
the LDFLAGS environment variable instead of forcing LDFLAGS to be
unset.  "configure --help" documents LDFLAGS as part of the standard
configure script calling convention.

Addresses-Sourceforge-Feature-Request: #1937287
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>