Binary caches on MacOS - allow expanded RPATHs (#11345)

Fixes #11335

Update the Spack compiler wrappers to add the headerpad_max_install_names
linker flag on MacOS. This allows the install_name_tool to rewrite
the RPATH entry of the binary to be longer if needed. This is
primarily useful for creating and distributing binary caches of
packages (i.e. using the "spack buildcache" command); binary caches
created on MacOS before this commit may not successfully relocate
(if the target root path is larger).

lib/spack/env/cc

@@ -382,6 +382,17 @@ case "$mode" in
         flags=("${flags[@]}" "${SPACK_LDFLAGS[@]}") ;;
 esac
 
+# On macOS insert headerpad_max_install_names linker flag
+if [[ ($mode == ld || $mode == ccld) && "$SPACK_SHORT_SPEC" =~ "darwin" ]];
+then
+    case "$mode" in
+        ld)
+            flags=("${flags[@]}" -headerpad_max_install_names) ;;
+        ccld)
+            flags=("${flags[@]}" -Wl,-headerpad_max_install_names) ;;
+    esac
+fi
+
 # Prepend include directories
 IFS=':' read -ra include_dirs <<< "$SPACK_INCLUDE_DIRS"
 if [[ $mode == cpp || $mode == cc || $mode == as || $mode == ccld ]]; then