/usr/lib/pkgconfig/fftw3l.pc is in libfftw3-dev 3.3-1ubuntu1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 10 | prefix=/usr
exec_prefix=${prefix}
libdir=${exec_prefix}/lib
includedir=${prefix}/include
Name: FFTW
Description: fast Fourier transform library
Version: 3.3
Libs: -L${libdir} -lfftw3l -lm
Cflags: -I${includedir}
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