<<Thanks again for your input,
The command I ran:

> ./configure --prefix=/home/pol/Downloads/sw/db/fb/inst --with-builtin-libtommath

then

> make

Same failure as before (at bottom).
I totally get your point about it being to do with libtommath, that much is obvious from the error messages.
However, I have tried the solution proposed - i.e. using ./configure with the --wtih-builtin-libtommath flag.
Now, I need further assistance. This is failing. I am compiling on Fedora 29 with Development Tools installed.
Where do I need to place, or what do I need to do, with the tommath code/library in order for a simple compile to work?
Any assistance much appreciated. >>

Look very carefully at the error you are getting and you will notice that although
you are specifying --with-builtin-libtommath
its actually picking up the non fPIC lib in /usr/local/lib

---> /usr/bin/ld: //usr/local/lib/libtommath.a
This is being picked up before the builtin one within extern
probably because of your library search paths

The correct library would be extern/libtommath/.libs for libtommath.so not libtommath.a
See gen/make.defaults

Paul




========== Fails =====================



/usr/bin/ld: //usr/local/lib/libtommath.a(bn_mp_read_radix.o): relocation R_X86_64_32S against symbol `mp_s_rmap_reverse' can not be used when making a shared object; recompile with -fPIC
/usr/bin/ld: //usr/local/lib/libtommath.a(bn_s_mp_exptmod.o): relocation R_X86_64_32 against symbol `mp_reduce' can not be used when making a shared object; recompile with -fPIC
/usr/bin/ld: //usr/local/lib/libtommath.a(bn_s_mp_exptmod_fast.o): relocation R_X86_64_32 against symbol `mp_reduce_2k' can not be used when making a shared object; recompile with -fPIC
/usr/bin/ld: final link failed: nonrepresentable section on output
collect2: error: ld returned 1 exit status
make[3]: *** [Makefile:305: /home/pol/Downloads/sw/db/fb/gen/Release/firebird/lib/libfbclient.so.3.0.4] Error 1
make[3]: Leaving directory '/home/p ol/Downloads/sw/db/fb/gen'
make[2]: *** [Makefile:180: master_process] Error 2
make[2]: Leaving directory '/home/pol/Downloads/sw/db/fb/gen'
make[1]: *** [Makefile:66: firebird] Error 2
make[1]: Leaving directory '/home/pol/Downloads/sw/db/fb/gen'
make: *** [Makefile:6: firebird] Error 2