make -lm a private library in fftw.pc.in

Julian Taylor says:

  Make -lm a private library, libfftw is linked against it so clients
  don't need it.  You can use pkg-configs --static flag for static
  linking.  this works on all systems where indirect linking works,
  probably on all others pkg-config is not supported anyway (wild
  guess).

fftw.pc.in

@@ -6,5 +6,6 @@ includedir=@includedir@
 Name: FFTW
 Description: fast Fourier transform library
 Version: @VERSION@
-Libs: -L${libdir} -lfftw3@PREC_SUFFIX@ @LIBQUADMATH@ -lm
+Libs: -L${libdir} -lfftw3@PREC_SUFFIX@ @LIBQUADMATH@
+Libs.private: -lm
 Cflags: -I${includedir}