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/gfx/main.cpp

@@ -1,5 +1,7 @@
 // SPDX-License-Identifier: MIT
 
+#include "platform.hpp"
+
 #include "gfx/main.hpp"
 
 #include <inttypes.h>
@@ -19,7 +21,6 @@
 #include "diagnostics.hpp"
 #include "file.hpp"
 #include "helpers.hpp"
-#include "platform.hpp"
 #include "style.hpp" // style_Parse
 #include "usage.hpp"
 #include "util.hpp"

src/gfx/pal_spec.cpp

@@ -1,5 +1,7 @@
 // SPDX-License-Identifier: MIT
 
+#include "platform.hpp"
+
 #include "gfx/pal_spec.hpp"
 
 #include <algorithm>
@@ -22,7 +24,6 @@
 #include "diagnostics.hpp"
 #include "file.hpp"
 #include "helpers.hpp"
-#include "platform.hpp"
 #include "util.hpp" // UpperMap, parseDigit
 
 #include "gfx/main.hpp"