Support selecting static builds for windows only

Cargo.toml

@@ -37,4 +37,9 @@ rmp-serde = "^0.13.7"
 benchmarks = []
 std = []
 default = ["serde", "std"]
+# use-pkg-config should be deprecated in the future and become the default
 use-pkg-config = ["libsodium-sys/use-pkg-config"]
+# always force a static build, even if use-pkg-config becomes the default
+static = ["libsodium-sys/static"]
+# only force a static build on windows
+static-windows = ["libsodium-sys/static-windows"]

libsodium-sys/Cargo.toml

@@ -31,4 +31,9 @@ libc = { version = "0.2" , default-features = false }
 name = "libsodium_sys"
 
 [features]
+# use-pkg-config should be deprecated in the future and become the default
 use-pkg-config = []
+# always force a static build, even if use-pkg-config becomes the default
+static = []
+# only force a static build on windows
+static-windows = []

libsodium-sys/build.rs

@@ -34,7 +34,7 @@ fn main() {
 
     let lib_dir_isset = env::var("SODIUM_LIB_DIR").is_ok();
     let use_pkg_isset = if cfg!(feature = "use-pkg-config") {
-        true
+        cfg!(not(feature = "static")) && cfg!(not(all(feature = "static-windows", windows)))
     } else {
         env::var("SODIUM_USE_PKG_CONFIG").is_ok()
     };