proc-macro-srv: make usage of RTLD_DEEPBIND portable

the constant is wrong on some platforms (e.g., on mips64el it's 0x10, and 0x8
is RTLD_NOLOAD which makes all this functionality broken), the libc crate takes
care of those differences for us.

fallback to not setting the flag in non-glibc environments - some of them might
have support for it using a different value that we don't know about, and some
of them lack it entirely.

Signed-off-by: Fabian Grünbichler <f.gruenbichler@proxmox.com>

src/tools/rust-analyzer/Cargo.lock

@@ -1377,6 +1377,7 @@ version = "0.0.0"
 dependencies = [
  "expect-test",
  "intern",
+ "libc",
  "libloading",
  "memmap2",
  "object 0.33.0",

src/tools/rust-analyzer/crates/proc-macro-srv/Cargo.toml

@@ -14,6 +14,7 @@ doctest = false
 
 [dependencies]
 object.workspace = true
+libc.workspace = true
 libloading.workspace = true
 memmap2.workspace = true
 

src/tools/rust-analyzer/crates/proc-macro-srv/src/dylib.rs

@@ -28,11 +28,16 @@ fn load_library(file: &Utf8Path) -> Result<Library, libloading::Error> {
 
 #[cfg(unix)]
 fn load_library(file: &Utf8Path) -> Result<Library, libloading::Error> {
+    // not defined by POSIX, different values on mips vs other targets
+    #[cfg(target_env = "gnu")]
+    use libc::RTLD_DEEPBIND;
     use libloading::os::unix::Library as UnixLibrary;
-    use std::os::raw::c_int;
+    // defined by POSIX
+    use libloading::os::unix::RTLD_NOW;
 
-    const RTLD_NOW: c_int = 0x00002;
-    const RTLD_DEEPBIND: c_int = 0x00008;
+    // MUSL and bionic don't have it..
+    #[cfg(not(target_env = "gnu"))]
+    const RTLD_DEEPBIND: std::os::raw::c_int = 0x0;
 
     unsafe { UnixLibrary::open(Some(file), RTLD_NOW | RTLD_DEEPBIND).map(|lib| lib.into()) }
 }