"MACOSX_DEPLOYMENT_TARGET"

[Ryan Schmidt]

With debug output on, I see this fly by during the configure phase:


DEBUG: Environment: CFLAGS='-O2' CPPFLAGS='-I/mp/include' CXXFLAGS='- 
O2' CPP='/usr/bin/cpp-4.0' CXX='/usr/bin/g++-4.0' F90FLAGS='-O2'  
LDFLAGS='-L/mp/lib' FCFLAGS='-O2' OBJC='/usr/bin/gcc-4.0' INSTALL='/ 
usr/bin/install -c' OBJCFLAGS='-O2' FFLAGS='-O2'  
"MACOSX_DEPLOYMENT_TARGET"='10.5' CC='/usr/bin/gcc-4.0'


Why is "MACOSX_DEPLOYMENT_TARGET" in quotes an the other environment  
variable names aren't?