I need to compile a standalone .a or .so library in Ubuntu 10.04 from the FFTW source code. I find that the directory layout of the source distribution is so complex that I have no idea where to start.
I need to use an older version of gcc as well.
If you just want the libraries install the libfftw3-dev package,
http://packages.ubuntu.com/lucid/amd64/libfftw3-dev/filelist
To build from source follow the trivial steps from the manual,
http://www.fftw.org/fftw3_doc/Installation-on-Unix.html#Installation-on-Unix
It's literally the basic installation of every piece of autoconf using software ever:
./configure
make
make install