Segu Venkatesh (itsvenky86@gmail.com) came across this error while trying to buildroot a new kernel on the gmp module:

checking size of unsigned short... 2
checking for unsigned... yes
checking size of unsigned... 4
checking for unsigned long... yes
checking size of unsigned long... 4
checking for mp_limb_t... yes
checking size of mp_limb_t... 4
checking for stack_t... yes
checking for tputs in -lncurses... yes
checking for readline in -lreadline... no
checking readline detected... no
checking for bison... bison -y
checking for flex... flex
checking for yywrap in -lfl... yes
checking lex output file root... configure: error: cannot find output from flex; giving up
make: *** [/home/stars/buildroot-2010.02/output/build/gmp-4.2.4/.configured] Error 1

Solution:
in the buildroot-2010.02/output/build/gmp-4.2.4 directory create a file conftest.l having :

%%
%%

as content and do a make again.