c89 compatibility

[stevengj]

Fixes #259, adds -std=c89 -pedantic -Werror to compilation flags on Travis to catch future C89 errors.

[jschueller]

can you include my appveyor changes ?

[jschueller]

maybe it's not necessary to build vs14 twice for 32 and 64 bits

[stevengj]

I would prefer to test both win32 and win64 with the same VS version.

[stevengj]

Changed the test to be a little less strict and allow POSIX 2001 functions, so that it allows functions like vsnprintf and isinf that are widely available even on C89-ish compilers. (I'm not willing to work very hard to support 20-year-old systems unless there is evidence that it is actually needed.)

[vhaguiar]

bobyqa seems unstable, given the same initial value, algorithm is deterministic, so it should be stable.