Compiler DEFINEs with string contents lead to invalid XCode pro jects

When using variables in the .pro file and passing them as DEFINEs to the compiler then this works fine on the command line on Linux and OSX. However, when creating an XCode project from the project file, XCode can't read the project, but aborts with:

2007-11-27 22:11:37.008 Xcode[18208] CFLog (0): CFMessagePort: bootstrap_register(): failed 1103 (0x44f), port = 0x3003, name = 'com.apple.Xcode.ServiceProvider'
See /usr/include/servers/bootstrap_defs.h for the error codes.
2007-11-27 22:11:37.010 Xcode[18208] CFLog (99): CFMessagePortCreateLocal(): failed to name Mach port (com.apple.Xcode.ServiceProvider)
2007-11-27 22:11:37.504 Xcode[18208] XML parser error:
Unexpected character / at line 1
Old-style plist parser error:
Expected terminating ')' for array at line 3827