Build static library with `-fPIC`

Trying to build Inkscape 1.x (master branch) with double-conversion, the [error below][1] is shown.

```
[ 87%] Linking CXX shared library ../lib/libinkscape_base.so
/usr/lib/gcc/x86_64-pc-linux-gnu/7.3.0/../../../../x86_64-pc-linux-gnu/bin/ld: /usr/lib/gcc/x86_64-pc-linux-gnu/7.3.0/../../../../lib64/libdouble-conversion.a(double-conversion.cc.o): relocation R_X86_64_32 against `.bss' can not be used when making a shared object; recompile with -fPIC
/usr/lib/gcc/x86_64-pc-linux-gnu/7.3.0/../../../../x86_64-pc-linux-gnu/bin/ld: /usr/lib/gcc/x86_64-pc-linux-gnu/7.3.0/../../../../lib64/libdouble-conversion.a(fast-dtoa.cc.o): relocation R_X86_64_32 against `.rodata' can not be used when making a shared object; recompile with -fPIC
/usr/lib/gcc/x86_64-pc-linux-gnu/7.3.0/../../../../x86_64-pc-linux-gnu/bin/ld: /usr/lib/gcc/x86_64-pc-linux-gnu/7.3.0/../../../../lib64/libdouble-conversion.a(fixed-dtoa.cc.o): relocation R_X86_64_32 against `.rodata' can not be used when making a shared object; recompile with -fPIC
/usr/lib/gcc/x86_64-pc-linux-gnu/7.3.0/../../../../x86_64-pc-linux-gnu/bin/ld: /usr/lib/gcc/x86_64-pc-linux-gnu/7.3.0/../../../../lib64/libdouble-conversion.a(strtod.cc.o): relocation R_X86_64_32 against `.rodata' can not be used when making a shared object; recompile with -fPIC
/usr/lib/gcc/x86_64-pc-linux-gnu/7.3.0/../../../../x86_64-pc-linux-gnu/bin/ld: /usr/lib/gcc/x86_64-pc-linux-gnu/7.3.0/../../../../lib64/libdouble-conversion.a(bignum.cc.o): relocation R_X86_64_32 against `.rodata' can not be used when making a shared object; recompile with -fPIC
/usr/lib/gcc/x86_64-pc-linux-gnu/7.3.0/../../../../x86_64-pc-linux-gnu/bin/ld: /usr/lib/gcc/x86_64-pc-linux-gnu/7.3.0/../../../../lib64/libdouble-conversion.a(bignum-dtoa.cc.o): relocation R_X86_64_32 against `.rodata' can not be used when making a shared object; recompile with -fPIC
/usr/lib/gcc/x86_64-pc-linux-gnu/7.3.0/../../../../x86_64-pc-linux-gnu/bin/ld: /usr/lib/gcc/x86_64-pc-linux-gnu/7.3.0/../../../../lib64/libdouble-conversion.a(cached-powers.cc.o): relocation R_X86_64_32 against `.rodata' can not be used when making a shared object; recompile with -fPIC
/usr/lib/gcc/x86_64-pc-linux-gnu/7.3.0/../../../../x86_64-pc-linux-gnu/bin/ld: final link failed: Nonrepresentable section on output
collect2: error: ld returned 1 exit status
```

Should double-conversion be built with `-fPIC` by default?

[1]: https://gitlab.com/inkscape/inkscape/issues/326