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/fix/fix.cpp

@@ -1,5 +1,7 @@
 // SPDX-License-Identifier: MIT
 
+#include "platform.hpp"
+
 #include "fix/fix.hpp"
 #include <sys/stat.h>
 
@@ -14,7 +16,6 @@
 
 #include "diagnostics.hpp"
 #include "helpers.hpp"
-#include "platform.hpp"
 #include "util.hpp" // xclose, xfclose
 
 #include "fix/main.hpp"

src/fix/main.cpp

@@ -1,5 +1,7 @@
 // SPDX-License-Identifier: MIT
 
+#include "platform.hpp"
+
 #include "fix/main.hpp"
 
 #include <errno.h>
@@ -15,7 +17,6 @@
 #include "cli.hpp"
 #include "diagnostics.hpp"
 #include "helpers.hpp"
-#include "platform.hpp"
 #include "style.hpp"
 #include "usage.hpp"
 #include "util.hpp"

src/fix/mbc.cpp

@@ -1,5 +1,7 @@
 // SPDX-License-Identifier: MIT
 
+#include "platform.hpp" // strcasecmp
+
 #include "fix/mbc.hpp"
 
 #include <optional>
@@ -10,8 +12,7 @@
 #include <unordered_map>
 #include <utility>
 
-#include "helpers.hpp"  // unreachable_
-#include "platform.hpp" // strcasecmp
+#include "helpers.hpp" // unreachable_
 #include "util.hpp"
 
 #include "fix/warning.hpp"