Improve handling of platform-specific build options (#2162)

* Opt into POSIX functions with `_POSIX_C_SOURCE` before any system
  headers are `#include`d
* Allow user to override `make develop`'s `WARNFLAGS` and `CXXFLAGS`
* Use `make develop` on 32-bit Cygwin with sanitizers disabled
* Do not unnecessarily redefine `fseek` and `ftell`
* Disable a false-positive `-Wno-null-dereference` on 32-bit Cygwin

Co-authored-by: ISSOtm <[email protected]>

include/file.hpp

@@ -3,6 +3,8 @@
 #ifndef RGBDS_FILE_HPP
 #define RGBDS_FILE_HPP
 
+#include "platform.hpp"
+
 #include <fcntl.h>
 #include <fstream>
 #include <ios>
@@ -12,7 +14,6 @@
 #include <variant>
 
 #include "helpers.hpp" // assume
-#include "platform.hpp"
 
 class File {
 	std::variant<std::streambuf *, std::filebuf> _file;