introduce buildcfg module and PROXMOX_CONFIGDIR

buildcfg.rs should contain convenience variables or macros
for using build-time configured variables

For now we replace hardcoded "/etc/proxmox-backup/<foo>"
with configdir!("<foo>").

Signed-off-by: Wolfgang Bumiller <w.bumiller@proxmox.com>

src/lib.rs

@@ -1,3 +1,6 @@
+#[macro_use]
+pub mod buildcfg;
+
 #[macro_use]
 pub mod tools;