Another round of performance tweaks for issue #3408.

These should make our igamma implementation comparable in performance to the dcdflib FORTRAN routine - at least as far as MSVC is concerned.

[SVN r56503]

libs/math

@@ -1 +1 @@
-Subproject commit 8418e9d68e5738c24268b3283b05b11fe6dab9a4
+Subproject commit 71940bec79ba9ad712d1481f23e4f1cb134d327f