fixes stack traces on osx

[dethrace-labs]

Stack traces work again with position independent code enabled.

******************
Caught SIGSEGV
******************

Stack trace:
0: GetALineAndDontArgue (in dethrace) (utility.c:325)
1: GetAFloat (in dethrace) (loading.c:2886)
2: LoadGeneralParameters (in dethrace) (loading.c:379)
3: InitialiseApplication (in dethrace) (init.c:356)
4: InitialiseDeathRace (in dethrace) (init.c:419)
5: GameMain (in dethrace) (main.c:123)
6: original_main (in dethrace) (win95sys.c:949)
7: main (in dethrace) (main.c:34)

inspired by https://github.com/mysql/mysql-server/blob/ea7087d885006918ad54458e7aad215b1650312c/storage/ndb/src/common/portlib/ndb_stacktrace.cpp#L76

Fixes #197