Allow easy overriding of the opt-level/debuginfo/codegen-units flags

Some crates use debuginfo=0 or debuginfo=1 to reduce memory pressure during
builds. This should make this easy.

Fixes https://pagure.io/fedora-rust/rust2rpm/issue/181

data/macros.rust

@@ -1,14 +1,18 @@
 %__rustc %{_bindir}/rustc
 %__rustdoc %{_bindir}/rustdoc
 
+%rustflags_opt_level 3
+%rustflags_debuginfo 2
+%rustflags_codegen_units 1
+
 # Enable optimization, debuginfo, and link hardening.
 %build_rustflags %{shrink:
-  -Copt-level=3
-  -Cdebuginfo=2
+  -Copt-level=%rustflags_opt_level
+  -Cdebuginfo=%rustflags_debuginfo
+  -Ccodegen-units=%rustflags_codegen_units
   -Clink-arg=-Wl,-z,relro
   -Clink-arg=-Wl,-z,now
   %{?_package_note_file:-Clink-arg=-Wl,-dT,%{_package_note_file}}
-  -Ccodegen-units=1
   --cap-lints=warn
 }