Use proper generator configuration in MinGW Windows toolchain

Author: Osspial

src/lib.rs

@@ -307,7 +307,21 @@ impl Config {
                 // studio build system but instead use makefiles that MinGW can
                 // use to build.
                 if self.generator.is_none() {
-                    cmd.arg("-G").arg("MSYS Makefiles");
+                    // If make.exe isn't found, that means we may be using a MinGW
+                    // toolchain instead of a MSYS2 toolchain. If neither is found,
+                    // the build cannot continue.
+                    let has_msys2 = Command::new("make").spawn().err()
+                        .map(|e| e.kind() != ErrorKind::NotFound).unwrap_or(true);
+                    let has_mingw32 = Command::new("mingw32-make").spawn().err()
+                        .map(|e| e.kind() != ErrorKind::NotFound).unwrap_or(true);
+
+                    let generator = match (has_msys2, has_mingw32) {
+                        (true, _) => "MSYS Makefiles",
+                        (false, true) => "MinGW Makefiles",
+                        (false, false) => fail("no valid generator found for GNU toolchain; MSYS or MinGW must be installed")
+                    };
+
+                    cmd.arg("-G").arg(generator);
                 }
             } else {
                 // If we're cross compiling onto windows, then set some