extension-functions.c: math.h errors and sqlite tcl parser errors on some builds

Issue #7 resolved
Carlin Desautels
created an issue

acosh, asinh, atanh, isblank are being overwritten on unix machines where math.h is available, resulting in a syntax error.

A comment following an include is creating a syntax error when using sqlite's tcl parser to build sqlite amalgamation

A definition of isnan is required to supported the extensions math functions on windows when visual studio is used to compile