diff --git a/secp256k1-sys/build.rs b/secp256k1-sys/build.rs
index 377c485..7347ccd 100644
--- a/secp256k1-sys/build.rs
+++ b/secp256k1-sys/build.rs
@@ -52,9 +52,10 @@ fn main() {
     #[cfg(feature = "recovery")]
     base_config.define("ENABLE_MODULE_RECOVERY", Some("1"));
 
-    // Header files. WASM only.
+    // WASM headers and size/align defines.
     if env::var("CARGO_CFG_TARGET_ARCH").unwrap() == "wasm32" {
-        base_config.include("wasm-sysroot");
+        base_config.include("wasm/wasm-sysroot")
+                   .file("wasm/wasm.c");
     }
 
     // secp256k1
diff --git a/secp256k1-sys/wasm-sysroot/stdio.h b/secp256k1-sys/wasm-sysroot/stdio.h
deleted file mode 100644
index 36cc900..0000000
--- a/secp256k1-sys/wasm-sysroot/stdio.h
+++ /dev/null
@@ -1,17 +0,0 @@
-#include <stddef.h>
-#define alignof(type) offsetof (struct { char c; type member; }, member)
-
-extern const unsigned char WASM32_INT_SIZE = sizeof(int);
-extern const unsigned char WASM32_INT_ALIGN = alignof(int);
-
-extern const unsigned char WASM32_UNSIGNED_INT_SIZE = sizeof(unsigned int);
-extern const unsigned char WASM32_UNSIGNED_INT_ALIGN = alignof(unsigned int);
-
-extern const unsigned char WASM32_SIZE_T_SIZE = sizeof(size_t);
-extern const unsigned char WASM32_SIZE_T_ALIGN = alignof(size_t);
-
-extern const unsigned char WASM32_UNSIGNED_CHAR_SIZE = sizeof(unsigned char);
-extern const unsigned char WASM32_UNSIGNED_CHAR_ALIGN = alignof(unsigned char);
-
-extern const unsigned char WASM32_PTR_SIZE = sizeof(void*);
-extern const unsigned char WASM32_PTR_ALIGN = alignof(void*);
\ No newline at end of file
diff --git a/secp256k1-sys/wasm-sysroot/stdlib.h b/secp256k1-sys/wasm/wasm-sysroot/stdio.h
similarity index 100%
rename from secp256k1-sys/wasm-sysroot/stdlib.h
rename to secp256k1-sys/wasm/wasm-sysroot/stdio.h
diff --git a/secp256k1-sys/wasm/wasm-sysroot/stdlib.h b/secp256k1-sys/wasm/wasm-sysroot/stdlib.h
new file mode 100644
index 0000000..e69de29
diff --git a/secp256k1-sys/wasm-sysroot/string.h b/secp256k1-sys/wasm/wasm-sysroot/string.h
similarity index 100%
rename from secp256k1-sys/wasm-sysroot/string.h
rename to secp256k1-sys/wasm/wasm-sysroot/string.h
diff --git a/secp256k1-sys/wasm/wasm.c b/secp256k1-sys/wasm/wasm.c
new file mode 100644
index 0000000..8eacae6
--- /dev/null
+++ b/secp256k1-sys/wasm/wasm.c
@@ -0,0 +1,17 @@
+#include <stddef.h>
+#define alignof(type) offsetof (struct { char c; type member; }, member)
+
+const unsigned char WASM32_INT_SIZE = sizeof(int);
+const unsigned char WASM32_INT_ALIGN = alignof(int);
+
+const unsigned char WASM32_UNSIGNED_INT_SIZE = sizeof(unsigned int);
+const unsigned char WASM32_UNSIGNED_INT_ALIGN = alignof(unsigned int);
+
+const unsigned char WASM32_SIZE_T_SIZE = sizeof(size_t);
+const unsigned char WASM32_SIZE_T_ALIGN = alignof(size_t);
+
+const unsigned char WASM32_UNSIGNED_CHAR_SIZE = sizeof(unsigned char);
+const unsigned char WASM32_UNSIGNED_CHAR_ALIGN = alignof(unsigned char);
+
+const unsigned char WASM32_PTR_SIZE = sizeof(void*);
+const unsigned char WASM32_PTR_ALIGN = alignof(void*);