------------- test.c ------------------------
#define TESTER( foo ) { .export TEST##foo; testing foo }

TESTER( A )
TESTER( A                               \
        B )
-----------------

"gcc-4.5 -x c -E tester.c -o tester.E && cat tester.E" yields:

{ .export TESTA; testing A }
{ .export
 TESTA
# 5 "tester.c"
 B; testing A B }

"gcc-4.2 -x c -E tester.c -o tester.E && cat tester.E" yields:
{ .export TESTA; testing A }
{ .export TESTA B; testing A B }