Allow SQLite to be linked against dynamically (#7057)

Keeping the default behaviour of SQLite being built statically,
so as not to break anyone's workflow, but allowing for downstream
packagers to link dynamically against SQLite (where it's fine because
that's the point of package managers).

Note that SQLite is still *not* enabled by default, thanks to the `?` operator.

Co-authored-by: Daniel García <dani-garcia@users.noreply.github.com>

build.rs

@@ -2,21 +2,21 @@ use std::env;
 use std::process::Command;
 
 fn main() {
-    // This allow using #[cfg(sqlite)] instead of #[cfg(feature = "sqlite")], which helps when trying to add them through macros
-    #[cfg(feature = "sqlite")]
+    // These allow using e.g. #[cfg(mysql)] instead of #[cfg(feature = "mysql")], which helps when trying to add them through macros
+    #[cfg(feature = "sqlite_system")] // The `sqlite` feature implies this one.
     println!("cargo:rustc-cfg=sqlite");
     #[cfg(feature = "mysql")]
     println!("cargo:rustc-cfg=mysql");
     #[cfg(feature = "postgresql")]
     println!("cargo:rustc-cfg=postgresql");
-    #[cfg(feature = "s3")]
-    println!("cargo:rustc-cfg=s3");
-
-    #[cfg(not(any(feature = "sqlite", feature = "mysql", feature = "postgresql")))]
+    #[cfg(not(any(feature = "sqlite_system", feature = "mysql", feature = "postgresql")))]
     compile_error!(
         "You need to enable one DB backend. To build with previous defaults do: cargo build --features sqlite"
     );
 
+    #[cfg(feature = "s3")]
+    println!("cargo:rustc-cfg=s3");
+
     // Use check-cfg to let cargo know which cfg's we define,
     // and avoid warnings when they are used in the code.
     println!("cargo::rustc-check-cfg=cfg(sqlite)");