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]>

.clang-format

@@ -42,6 +42,8 @@ EmptyLineBeforeAccessModifier: Leave
 FixNamespaceComments: false
 IncludeBlocks: Regroup
 IncludeCategories:
+  - Regex: '^"platform\.hpp"$'
+    Priority: -1
   - Regex: '^<sys/'
     Priority: 0
   - Regex: '^<'