We enable -D_FORTIFY_SOURCE=2 by default, but with 4.9.0 we're encountering an error while building libitm.  It can be reproduced without modifying the compiler by configuring with:

CXXFLAGS="-O2 -g -D_FORTIFY_SOURCE=2" ../dist/gcc/gcc-4.9.0/configure --enable-languages=c,c++ --disable-werror --disable-bootstrap

The error is:

libtool: compile:  /home/dirtyepic/build/./gcc/xg++ -B/home/dirtyepic/build/./gcc/ -nostdinc++ -nostdinc++ -I/home/dirtyepic/build/x86_64-unknown-linux-gnu/libstdc++-v3/include/x86_64-unknown-linux-gnu -I/home/dirtyepic/build/x86_64-unknown-linux-gnu/libstdc++-v3/include -I/home/dirtyepic/dist/gcc/gcc-4.9.0/libstdc++-v3/libsupc++ -I/home/dirtyepic/dist/gcc/gcc-4.9.0/libstdc++-v3/include/backward -I/home/dirtyepic/dist/gcc/gcc-4.9.0/libstdc++-v3/testsuite/util -L/home/dirtyepic/build/x86_64-unknown-linux-gnu/libstdc++-v3/src -L/home/dirtyepic/build/x86_64-unknown-linux-gnu/libstdc++-v3/src/.libs -L/home/dirtyepic/build/x86_64-unknown-linux-gnu/libstdc++-v3/libsupc++/.libs -B/usr/local/x86_64-unknown-linux-gnu/bin/ -B/usr/local/x86_64-unknown-linux-gnu/lib/ -isystem /usr/local/x86_64-unknown-linux-gnu/include -isystem /usr/local/x86_64-unknown-linux-gnu/sys-include -DHAVE_CONFIG_H -I. -I../../../dist/gcc/gcc-4.9.0/libitm -I../../../dist/gcc/gcc-4.9.0/libitm/config/linux/x86 -I../../../dist/gcc/gcc-4.9.0/libitm/config/linux -I../../../dist/gcc/gcc-4.9.0/libitm/config/x86 -I../../../dist/gcc/gcc-4.9.0/libitm/config/posix -I../../../dist/gcc/gcc-4.9.0/libitm/config/generic -I../../../dist/gcc/gcc-4.9.0/libitm -mrtm -Wall -pthread -Werror -std=gnu++0x -funwind-tables -fno-exceptions -fno-rtti -fabi-version=4 -O2 -g -D_FORTIFY_SOURCE=2 -D_GNU_SOURCE -MT util.lo -MD -MP -MF .deps/util.Tpo -c ../../../dist/gcc/gcc-4.9.0/libitm/util.cc  -fPIC -DPIC -o .libs/util.o

In file included from /usr/include/stdio.h:937:0,
                 from ../../../dist/gcc/gcc-4.9.0/libitm/util.cc:27:
/usr/include/bits/stdio2.h: In function ‘void GTM::gtm_verror(const char*, __va_list_tag*)’:
/usr/include/bits/stdio2.h:124:1: error: inlining failed in call to always_inline ‘int vfprintf(FILE*, const char*, __va_list_tag*)’: function body can be overwritten at link time
 vfprintf (FILE *__restrict __stream,
 ^
../../../dist/gcc/gcc-4.9.0/libitm/util.cc:35:31: error: called from here
   vfprintf (stderr, fmt, list);
                               ^

Comparing the preprocessor output for util.cc and a couple other C++ files that use vfprintf the most obvious difference is the declaration is missing __inline, ie.

extern __attribute__((always_inline)) __attribute__ ((__gnu_inline__)) __attribute__ ((__artificial__)) int vfprintf

instead of

extern __inline __attribute__ ((__always_inline__)) __attribute__ ((__gnu_inline__)) __attribute__ ((__artificial__)) int vfprintf