Configure et al: split up the lflags configuration item into two

The lflags configuration had a weird syntax with a % as separator.  If
it was present, whatever came before ended up as PEX_LIBS in Makefile
(usually, this is LDFLAGS), while whatever came after ended up as
EX_LIBS.

This change splits that item into lflags and ex_libs, making their use
more explicit.

Also, PEX_LIBS in all the Makefiles are renamed to LDFLAGS.

Reviewed-by: Rich Salz <rsalz@openssl.org>