libelf: fix build with clang16+ (#38915)

libelf fails to build with clang16+ due to Wimplicit-int and
Wimplicit-function-declarations becoming errors by default. This breaks
the configuration stage, so no build takes place. This patch fixes this
by passing -Wno-error=implicit-int and
-Wno-error=implicit-function-declarations as cflags.

var/spack/repos/builtin/packages/libelf/package.py

@@ -50,3 +50,10 @@ def configure_args(self):
 
     def install(self, spec, prefix):
         make("install", parallel=False)
+
+    def flag_handler(self, name, flags):
+        if name == "cflags":
+            if self.spec.satisfies("%clang@16:"):
+                flags.append("-Wno-error=implicit-int")
+                flags.append("-Wno-error=implicit-function-declaration")
+        return (flags, None, None)