Prevents errpr:
Android: The API specified by CMAKE_SYSTEM_VERSION='' is not an integer.

--- a/builder/cmake_builder.rs
+++ b/builder/cmake_builder.rs
@@ -233,6 +233,7 @@ impl CmakeBuilder {
         } else {
             emit_warning("ANDROID_STANDALONE_TOOLCHAIN not set.");
         }
+        _cmake_cfg.define("CMAKE_SYSTEM_VERSION", "@TERMUX_PKG_API_LEVEL@");
     }
 
     fn configure_windows(&self, cmake_cfg: &mut cmake::Config) {