lang/smlnj: Respect LDFLAGS and let LLD_UNSAFE actually work

It currently still tries to use lld.

/usr/bin/ld: error: can't create dynamic relocation R_386_32 against symbol: __stderrp in readonly segment; recompile object files with -fPIC

defined in /usr/lib32/libc.so.7
referenced by main.c

main.o:(main)

/usr/bin/ld: error: can't create dynamic relocation R_386_32 against symbol: DebugF in readonly segment; recompile object files with -fPIC

defined in main.o
referenced by main.c

main.o:(main)

http://beefy12.nyi.freebsd.org/data/head-amd64-default/p474314_s336154/logs/smlnj-110.77.log

PR: 226991
Reported by: emaste, pkg-fallout