refactor: Replace custom __USING_WINDOWS__ with WIN32 in subprocess.hpp

The `WIN32` macro is used across the entire code base to designate
building for Windows.

src/util/subprocess.hpp

@@ -53,16 +53,12 @@ Documentation for C++ subprocessing library.
 #include <string>
 #include <vector>
 
-#if (defined _MSC_VER) || (defined __MINGW32__)
-  #define __USING_WINDOWS__
-#endif
-
-#ifdef __USING_WINDOWS__
+#ifdef WIN32
   #include <codecvt>
 #endif
 
 extern "C" {
-#ifdef __USING_WINDOWS__
+#ifdef WIN32
   #include <windows.h>
   #include <io.h>
   #include <cwchar>
@@ -157,7 +153,7 @@ class OSError: public std::runtime_error
 //--------------------------------------------------------------------
 
 //Environment Variable types
-#ifndef __USING_WINDOWS__
+#ifndef WIN32
 	using env_string_t = std::string;
 	using env_char_t = char;
 #else
@@ -236,7 +232,7 @@ namespace util
     }
   }
 
-#ifdef __USING_WINDOWS__
+#ifdef WIN32
   inline std::string get_last_error(DWORD errorMessageID)
   {
     if (errorMessageID == 0)
@@ -451,7 +447,7 @@ namespace util
   }
 
 
-#ifndef __USING_WINDOWS__
+#ifndef WIN32
   /*!
    * Function: set_clo_on_exec
    * Sets/Resets the FD_CLOEXEC flag on the provided file descriptor
@@ -539,7 +535,7 @@ namespace util
   static inline
   int read_atmost_n(FILE* fp, char* buf, size_t read_upto)
   {
-#ifdef __USING_WINDOWS__
+#ifdef WIN32
     return (int)fread(buf, 1, read_upto, fp);
 #else
     int fd = fileno(fp);
@@ -612,7 +608,7 @@ namespace util
     return total_bytes_read;
   }
 
-#ifndef __USING_WINDOWS__
+#ifndef WIN32
   /*!
    * Function: wait_for_child_exit
    * Waits for the process with pid `pid` to exit
@@ -791,7 +787,7 @@ struct input
   }
   explicit input(IOTYPE typ) {
     assert (typ == PIPE && "STDOUT/STDERR not allowed");
-#ifndef __USING_WINDOWS__
+#ifndef WIN32
     std::tie(rd_ch_, wr_ch_) = util::pipe_cloexec();
 #endif
   }
@@ -824,7 +820,7 @@ struct output
   }
   explicit output(IOTYPE typ) {
     assert (typ == PIPE && "STDOUT/STDERR not allowed");
-#ifndef __USING_WINDOWS__
+#ifndef WIN32
     std::tie(rd_ch_, wr_ch_) = util::pipe_cloexec();
 #endif
   }
@@ -856,7 +852,7 @@ struct error
   explicit error(IOTYPE typ) {
     assert ((typ == PIPE || typ == STDOUT) && "STDERR not allowed");
     if (typ == PIPE) {
-#ifndef __USING_WINDOWS__
+#ifndef WIN32
       std::tie(rd_ch_, wr_ch_) = util::pipe_cloexec();
 #endif
     } else {
@@ -1159,7 +1155,7 @@ class Streams
   std::shared_ptr<FILE> output_ = nullptr;
   std::shared_ptr<FILE> error_  = nullptr;
 
-#ifdef __USING_WINDOWS__
+#ifdef WIN32
   HANDLE g_hChildStd_IN_Rd = nullptr;
   HANDLE g_hChildStd_IN_Wr = nullptr;
   HANDLE g_hChildStd_OUT_Rd = nullptr;
@@ -1266,7 +1262,7 @@ class Popen
 /*
   ~Popen()
   {
-#ifdef __USING_WINDOWS__
+#ifdef WIN32
     CloseHandle(this->process_handle_);
 #endif
   }
@@ -1342,7 +1338,7 @@ class Popen
 private:
   detail::Streams stream_;
 
-#ifdef __USING_WINDOWS__
+#ifdef WIN32
   HANDLE process_handle_;
   std::future<void> cleanup_future_;
 #endif
@@ -1407,7 +1403,7 @@ inline void Popen::start_process() noexcept(false)
 
 inline int Popen::wait() noexcept(false)
 {
-#ifdef __USING_WINDOWS__
+#ifdef WIN32
   int ret = WaitForSingleObject(process_handle_, INFINITE);
 
   return 0;
@@ -1428,7 +1424,7 @@ inline int Popen::wait() noexcept(false)
 
 inline int Popen::poll() noexcept(false)
 {
-#ifdef __USING_WINDOWS__
+#ifdef WIN32
   int ret = WaitForSingleObject(process_handle_, 0);
   if (ret != WAIT_OBJECT_0) return -1;
 
@@ -1478,7 +1474,7 @@ inline int Popen::poll() noexcept(false)
 
 inline void Popen::kill(int sig_num)
 {
-#ifdef __USING_WINDOWS__
+#ifdef WIN32
   if (!TerminateProcess(this->process_handle_, (UINT)sig_num)) {
     throw OSError("TerminateProcess", 0);
   }
@@ -1491,7 +1487,7 @@ inline void Popen::kill(int sig_num)
 
 inline void Popen::execute_process() noexcept(false)
 {
-#ifdef __USING_WINDOWS__
+#ifdef WIN32
   if (this->shell_) {
     throw OSError("shell not currently supported on windows", 0);
   }
@@ -1719,7 +1715,7 @@ namespace detail {
 
 
   inline void Child::execute_child() {
-#ifndef __USING_WINDOWS__
+#ifndef WIN32
     int sys_ret = -1;
     auto& stream = parent_->stream_;
 
@@ -1818,7 +1814,7 @@ namespace detail {
 
   inline void Streams::setup_comm_channels()
   {
-#ifdef __USING_WINDOWS__
+#ifdef WIN32
     util::configure_pipe(&this->g_hChildStd_IN_Rd, &this->g_hChildStd_IN_Wr, &this->g_hChildStd_IN_Wr);
     this->input(util::file_from_handle(this->g_hChildStd_IN_Wr, "w"));
     this->write_to_child_ = _fileno(this->input());