[MinGW] Missing option for static-linking libstdc++ (self-contained .exe)

[olekolek1000]

Describe the bug
Currently, imgui links with shared library libstdc++ and libgcc, causing the executable to not be portable (useful in making installers or portable apps). There should be an option to static-link stdc++ with the libcimgui.a.

To Reproduce

1. Compile executable with imgui-rs and statically-compiled SDL2
2. Try to run it outside development environment
3. "Cannot find libstdc++-6.dll"

Expected behavior
Application runs without any external dependencies

Please describe your environment

• imgui-rs main
• Microsoft Windows (MSYS2 MinGW UCRT64)

Other information
Working fix (needs customization, wip)

diff --git a/imgui-sdl2-support/Cargo.toml b/imgui-sdl2-support/Cargo.toml
index 5935df8..08a4301 100644
--- a/imgui-sdl2-support/Cargo.toml
+++ b/imgui-sdl2-support/Cargo.toml
@@ -12,7 +12,7 @@ categories = ["gui"]

 [dependencies]
 imgui = { version = "0.11.0", path = "../imgui" }
-sdl2 = "0.36.0"
+sdl2 = { version = "0.36.0", features = ["bundled", "static-link"] }

 [dev-dependencies]
 glow = "0.12.0"
diff --git a/imgui-sys/build.rs b/imgui-sys/build.rs
index 0951589..ffb9d7f 100644
--- a/imgui-sys/build.rs
+++ b/imgui-sys/build.rs
@@ -58,10 +58,10 @@ fn main() -> std::io::Result<()> {
     if !wasm_enabled {
         // C++ compiler
         let mut build = cc::Build::new();
-        build.cpp(true);
+        build.cpp(false);

         // imgui uses C++11 stuff from v1.87 onwards
-        build.flag_if_supported("-std=c++11");
+        build.flag("-std=gnu++11");

         // Set defines for compiler
         for (key, value) in DEFINES.iter() {
@@ -104,11 +104,11 @@ fn main() -> std::io::Result<()> {
         // Avoid the if-supported flag functions for easy cases, as they're
         // kinda costly.
         if compiler.is_like_gnu() || compiler.is_like_clang() {
-            build.flag("-fno-exceptions").flag("-fno-rtti");
+            build.flag("-fno-exceptions").flag("-fno-rtti").flag("-fno-threadsafe-statics");
         }

         // Build imgui lib, suppressing warnings.
-        build.warnings(false).file(imgui_cpp).compile("libcimgui.a");
+        build.warnings(false).file(imgui_cpp).static_flag(true).shared_flag(false).compile("libcimgui.a");
     }
     Ok(())
 }