It's actually not Arduino IDE 1.6.9 that did it, it was an update of Arduino AVR Boards you must have done and forgotten about because the toolchain included with Arduino IDE 1.6.9 doesn't contain that macro but Arduino AVR Boards 1.6.12 and higher does.
It's at line 948 of C:\Documents and Settings\Chris\Local Settings\Application Data\Arduino15\packages\arduino\tools\avr-gcc\4.9.2-atmel3.5.3-arduino2\avr\include\stdio.h:
#define BUFSIZ 1024
which causes your code to translate to:
const int 1024=90;
after the preprocessor is done with it so you can see the reason for the error.
In general I'd recommend reserving all caps for macros. The Arduino style is to use camel case for constants, variables and functions. This will reduce the chances for conflict. Caps are more work to type anyway.