I might be missing something obvious here. I want to be able to build MacRuby with debug information by tweaking the command-line instead of the code itself (which is what I had been doing until now). Something like: [last: 41] marca@SCML-MarcA:~/dev/git-repos/MacRuby$ CFLAGS="-DROXOR_VM_DEBUG" rake (in /Users/marca/dev/git-repos/MacRuby) /usr/bin/g++ -I. -I./include -fblocks -g -Wall -Wno-deprecated-declarations -Werror -arch x86_64 -DROXOR_VM_DEBUG -I/usr/local/include -D_DEBUG -D_GNU_SOURCE -D__STDC_LIMIT_MACROS -D__STDC_CONSTANT_MACROS -O3 -fno-rtti -fno-common -Woverloaded-virtual -I./icu-1060 -c vm.cpp -o .objs/vm.o vm.cpp:9:1: error: "ROXOR_VM_DEBUG" redefined <command-line>: error: this is the location of the previous definition rake aborted! Command failed with status (1): [/usr/bin/g++ -I. -I./include -fblocks -g -...] (See full trace by running task with --trace) This fails because vm.cpp has: #define ROXOR_VM_DEBUG 0 I could fix this by changing that to: #ifndef ROXOR_VM_DEBUG #define ROXOR_VM_DEBUG 0 #endif but the fact that it's not already there makes me think that people have another, easier way to switch compile flags. If there's an easier way, please let me know what it is and if there's not, should I send a pull request that wraps all the #defines in #ifndefs? Marc