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]>

src/usage.cpp

@@ -1,5 +1,7 @@
 // SPDX-License-Identifier: MIT
 
+#include "platform.hpp"
+
 #include "usage.hpp"
 
 #include <stdarg.h>
@@ -8,7 +10,6 @@
 
 #include "diagnostics.hpp" // vfatalx
 #include "helpers.hpp"
-#include "platform.hpp"
 #include "style.hpp"
 #include "util.hpp" // parseWholeNumber
 #include "version.hpp"