Fix build for CYGWIN

Attached patch fixes build on CYGWIN.
Hopefully, the patch is small and easy because it is just needed to add CYGWIN to the list of existing UNIX platforms.

Author: carlo-bramini

CMakeLists.txt

@@ -108,7 +108,7 @@ elseif(APPLE)
     if (CMAKE_SYSTEM_NAME STREQUAL "Darwin")
         target_compile_definitions(platform_wsi INTERFACE VK_USE_PLATFORM_MACOS_MVK)
     endif()
-elseif(CMAKE_SYSTEM_NAME MATCHES "Linux|BSD|DragonFly|GNU")
+elseif(CMAKE_SYSTEM_NAME MATCHES "Linux|BSD|DragonFly|GNU|CYGWIN")
     option(BUILD_WSI_XCB_SUPPORT "Build XCB WSI support" ON)
     option(BUILD_WSI_XLIB_SUPPORT "Build Xlib WSI support" ON)
     option(BUILD_WSI_XLIB_XRANDR_SUPPORT "Build X11 Xrandr WSI support" ON)

loader/vk_loader_platform.h

@@ -44,7 +44,7 @@
 
 // Set of platforms with a common set of functionality which is queried throughout the program
 #if defined(__linux__) || defined(__APPLE__) || defined(__Fuchsia__) || defined(__QNX__) || defined(__FreeBSD__) || \
-    defined(__OpenBSD__) || defined(__NetBSD__) || defined(__DragonFly__) || defined(__GNU__)
+    defined(__OpenBSD__) || defined(__NetBSD__) || defined(__DragonFly__) || defined(__GNU__) || defined(__CYGWIN__)
 #define COMMON_UNIX_PLATFORMS 1
 #else
 #define COMMON_UNIX_PLATFORMS 0
@@ -298,7 +298,7 @@ static inline char *loader_platform_dirname(char *path) { return dirname(path);
 
 // loader_platform_executable_path finds application path + name.
 // Path cannot be longer than 1024, returns NULL if it is greater than that.
-#if defined(__linux__) || defined(__GNU__)
+#if defined(__linux__) || defined(__GNU__) || defined(__CYGWIN__)
 static inline char *loader_platform_executable_path(char *buffer, size_t size) {
     ssize_t count = readlink("/proc/self/exe", buffer, size);
     if (count == -1) return NULL;