Fixed a build issue with non-Cygwin GCC under Windows.

docs/CHANGELOG.txt

@@ -34,6 +34,7 @@ HOW TO UPDATE?
 
 Other Changes:
 
+- Fixed a build issue with non-Cygwin GCC under Windows.
 - Examples: OpenGL3: Fixed error condition when using the GLAD loader. (#2059, #2002). [@jiri]
 
 

imgui.cpp

@@ -1506,7 +1506,7 @@ int ImTextCountUtf8BytesFromStr(const ImWchar* in_text, const ImWchar* in_text_e
 
 FILE* ImFileOpen(const char* filename, const char* mode)
 {
-#if defined(_WIN32) && !defined(__CYGWIN__)
+#if defined(_WIN32) && !defined(__CYGWIN__) && !defined(__GNUC__)
     // We need a fopen() wrapper because MSVC/Windows fopen doesn't handle UTF-8 filenames. Converting both strings from UTF-8 to wchar format (using a single allocation, because we can)
     const int filename_wsize = ImTextCountCharsFromUtf8(filename, NULL) + 1;
     const int mode_wsize = ImTextCountCharsFromUtf8(mode, NULL) + 1;