* sysdeps/ieee754/dbl-64/e_lgamma_r.c: Fix *signgamp for -0.0.
* sysdeps/ieee754/flt-32/e_lgammaf_r.c: Likewise.
* sysdeps/ieee754/ldbl-96/e_lgammal_r.c: Likewise.
* math/libm-test.inc: Add test for this case.
Half the patch by Christian Iseli <christian.iseli@licr.org>.