configure: document the use of the LD variable

configure.ac

@@ -529,6 +529,7 @@ FP_SET_CFLAGS_C99([CC],[CONF_CC_OPTS_STAGE2],[CONF_CPP_OPTS_STAGE2])
 
 dnl ** Which ld to use
 dnl --------------------------------------------------------------
+AC_ARG_VAR(LD,[Use as the path to ld. See also --disable-ld-override.])
 FIND_LD([$target],[GccUseLdOpt])
 CONF_GCC_LINKER_OPTS_STAGE1="$CONF_GCC_LINKER_OPTS_STAGE1 $GccUseLdOpt"
 CONF_GCC_LINKER_OPTS_STAGE2="$CONF_GCC_LINKER_OPTS_STAGE2 $GccUseLdOpt"