build: Add -U__STRICT_ANSI__ to CFLAGS on Cygwin

Cygwin's libc (newlib) doesn't obey a lot of unix feature test macros,
including _GNU_SOURCE; as a result, a lot of functions and defines get
masked out -- important defines such as M_PI and strcasecmp. Work around
it by undefining __STRICT_ANSI__ on cygwin systems.

This will still cause compilation issues on any non-cygwin system that
uses newlib, but hopefully nobody does that, or if they do, they will
find this commit message and know to add -U__STRICT_ANSI__ to their
CFLAGS. Hopefully.

waftools/detections/compiler.py

@@ -42,6 +42,7 @@ def __add_mingw_flags__(ctx):
 
 def __add_cygwin_flags__(ctx):
     ctx.env.CFLAGS += ['-mwin32']
+    ctx.env.CFLAGS += ['-U__STRICT_ANSI__']
 
 __compiler_map__ = {
     '__GNUC__':  __add_gcc_flags__,