Fix package detection and build when cross-compiling from MSVC to GNU

I and @dragonmux found two issues when building libusb from a MSVC host
to a MinGW target. When the build.rs script is built for the MSVC host,

- the vcpkg crate is used for detection (which correctly detects the
  MinGW target and reports an unsupported ABI)
- a MSVC flag is applied to the build (and thus breaking the build
  altogether)

This commit fixes both issues by changing vcpkg to a Windows-wide
dependency, and then if CARGO_CFG_TARGET_ENV is indeed MSVC, it is used
and the /source-charset:utf-8 flag is applied.

libusb1-sys/Cargo.toml

@@ -39,7 +39,7 @@ vendored = []
 [dependencies]
 libc = "0.2"
 
-[target.'cfg(target_env = "msvc")'.build-dependencies]
+[target.'cfg(target_os = "windows")'.build-dependencies]
 vcpkg = "0.2"
 
 [build-dependencies]

libusb1-sys/build.rs

@@ -19,17 +19,6 @@ pub fn link_framework(name: &str) {
     println!("cargo:rustc-link-lib=framework={}", name);
 }
 
-#[cfg(target_env = "msvc")]
-fn find_libusb_pkg(_statik: bool) -> bool {
-    match vcpkg::Config::new().find_package("libusb") {
-        Ok(_) => true,
-        Err(e) => {
-            println!("Can't find libusb pkg: {:?}", e);
-            false
-        }
-    }
-}
-
 fn get_macos_major_version() -> Option<usize> {
     if !cfg!(target_os = "macos") {
         return None;
@@ -45,8 +34,17 @@ fn get_macos_major_version() -> Option<usize> {
     Some(major)
 }
 
-#[cfg(not(target_env = "msvc"))]
 fn find_libusb_pkg(statik: bool) -> bool {
+    if std::env::var("CARGO_CFG_TARGET_ENV") == Ok("msvc".into()) {
+        #[cfg(target_os = "windows")]
+        return match vcpkg::Config::new().find_package("libusb") {
+            Ok(_) => true,
+            Err(e) => {
+                println!("Can't find libusb pkg: {:?}", e);
+                false
+            }
+        };
+    }
     // https://github.com/rust-lang/rust/issues/96943
     let needs_rustc_issue_96943_workaround: bool = get_macos_major_version()
         .map(|major| major >= 11)
@@ -171,8 +169,9 @@ fn make_source() {
     }
 
     if std::env::var("CARGO_CFG_TARGET_OS") == Ok("windows".into()) {
-        #[cfg(target_env = "msvc")]
-        base_config.flag("/source-charset:utf-8");
+        if std::env::var("CARGO_CFG_TARGET_ENV") == Ok("msvc".into()) {
+            base_config.flag("/source-charset:utf-8");
+        }
 
         base_config.warnings(false);
         base_config.define("OS_WINDOWS", Some("1"));