fftw: compile with -fPIC instead of -fPIE

[daniestevez]

Compiling with -fPIE doesn't allow building dynamic libraries (for instance, a dynamic library that contains blocks using fftw or a Python module with Python bindings for these blocks).

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Context: I'm building a Python module with Python bindings for gr4-packet-modem, and with -fPIE I get the following error:

[ 50%] Linking CXX shared module gr4_packet_modem_python.cpython-312-x86_64-linux-gnu.so
/usr/bin/ld: /gr4-packet-modem/build/gnuradio4/fftw/install/lib/libfftw3f.a(tensor7.o): warning: relocation against `fftwf_dimcmp' in read-only section `.text'
/usr/bin/ld: /gr4-packet-modem/build/gnuradio4/fftw/install/lib/libfftw3f.a(tensor7.o): relocation R_X86_64_PC32 against symbol `fftwf_dimcmp' can not be used when making a shared object; recompile with -fPIC
/usr/bin/ld: final link failed: bad value
c++: error: linker command failed with exit code 1 (use -v to see invocation)
make[2]: *** [CMakeFiles/gr4_packet_modem_python.dir/build.make:1013: gr4_packet_modem_python.cpython-312-x86_64-linux-gnu.so] Error 1
make[1]: *** [CMakeFiles/Makefile2:1022: CMakeFiles/gr4_packet_modem_python.dir/all] Error 2

With -fPIC the error goes away.

[drslebedev]

Thanks a lot for your PR!

Changes look good to me and the PR can be merged.