rustbuild: allow dynamically linking LLVM

The makefiles and `mklldeps.py` called `llvm-config --shared-mode` to
find out if LLVM defaulted to shared or static libraries, and just went
with that.  But under rustbuild, `librustc_llvm/build.rs` was assuming
that LLVM should be static, and even forcing `--link-static` for 3.9+.

Now that build script also uses `--shared-mode` to learn the default,
which should work better for pre-3.9 configured for dynamic linking, as
it wasn't possible back then to choose differently via `llvm-config`.

Further, the configure script now has a new `--enable-llvm-link-shared`
option, which allows one to manually override `--link-shared` on 3.9+
instead of forcing static.

src/bootstrap/compile.rs

@@ -212,6 +212,9 @@ pub fn rustc<'a>(build: &'a Build, target: &str, compiler: &Compiler<'a>) {
         cargo.env("LLVM_STATIC_STDCPP",
                   compiler_file(build.cxx(target), "libstdc++.a"));
     }
+    if build.config.llvm_link_shared {
+        cargo.env("LLVM_LINK_SHARED", "1");
+    }
     if let Some(ref s) = build.config.rustc_default_linker {
         cargo.env("CFG_DEFAULT_LINKER", s);
     }

src/bootstrap/config.rs

@@ -53,6 +53,7 @@ pub struct Config {
     pub llvm_release_debuginfo: bool,
     pub llvm_version_check: bool,
     pub llvm_static_stdcpp: bool,
+    pub llvm_link_shared: bool,
 
     // rust codegen options
     pub rust_optimize: bool,
@@ -343,6 +344,7 @@ impl Config {
                 ("OPTIMIZE_LLVM", self.llvm_optimize),
                 ("LLVM_VERSION_CHECK", self.llvm_version_check),
                 ("LLVM_STATIC_STDCPP", self.llvm_static_stdcpp),
+                ("LLVM_LINK_SHARED", self.llvm_link_shared),
                 ("OPTIMIZE", self.rust_optimize),
                 ("DEBUG_ASSERTIONS", self.rust_debug_assertions),
                 ("DEBUGINFO", self.rust_debuginfo),