TOMS344 is a FORTRAN77 library which implements ACM TOMS algorithm 344, for evaluating Student's T distribution.
The text of many ACM TOMS algorithms is available online through ACM: http://www.acm.org/pubs/calgo or NETLIB: http://www.netlib.org/toms/index.html.
TOMS344 is available in a FORTRAN77 version.
DCDFLIB, a FORTRAN90 library which evaluates many statistical distributions.
PROB, a FORTRAN90 library which evaluates many statistical distributions.
TEST_VALUES, a FORTRAN77 library which includes selected tabulated values of many mathematical functions.
You can go up one level to the FORTRAN77 source codes.