mk: Make nightlyism a configure option

configure

@@ -383,6 +383,7 @@ opt local-rust 0 "use an installed rustc rather than downloading a snapshot"
 opt pax-flags 0 "apply PaX flags to rustc binaries (required for GRSecurity/PaX-patched kernels)"
 opt inject-std-version 1 "inject the current compiler version of libstd into programs"
 opt rpath 1 "build rpaths into rustc itself"
+opt nightly 0 "build nightly packages"
 valopt prefix "/usr/local" "set installation prefix"
 valopt local-rust-root "/usr/local" "set prefix for local rust binary"
 valopt llvm-root "" "set LLVM root"

mk/main.mk

@@ -16,7 +16,7 @@
 CFG_RELEASE_NUM=0.10
 CFG_RELEASE_LABEL=-pre
 
-ifndef CFG_NIGHTLY
+ifndef CFG_ENABLE_NIGHTLY
 # This is the normal version string
 CFG_RELEASE=$(CFG_RELEASE_NUM)$(CFG_RELEASE_LABEL)
 CFG_PACKAGE_VERS=$(CFG_RELEASE)