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/buildcfg.rs

@@ -0,0 +1,6 @@
+pub const CONFIGDIR: &'static str = env!("PROXMOX_CONFIGDIR");
+
+#[macro_export]
+macro_rules! configdir {
+    ($subdir:expr) => (concat!(env!("PROXMOX_CONFIGDIR"), $subdir))
+}