configure fix for speex
On some configurations, the configure script errors out with: .configure: line 12024: syntax error near unexpected token 'FFT,' .configure: line 12024: ' PKG_CHECK_MODULES(FFT, fftw3f)'
see Gentoo bug #332861 at: https://bugs.gentoo.org/show_bug.cgi?id=332861
This patch modifies configure.ac and seems to fix the problem.