Add -fPIC to the static library

[jmcarcell]

This is already done for the dynamic library but not for the static one. Currently, building ROOT (https://github.com/root-project/root) when using the static library fails with:

[ 59%] Linking CXX shared library ../lib/libCore.so
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/build/jenkins/workspace/lcg_nightly_pipeline/install/dev3/pcre2/10.42/x86_64-el9-gcc15-opt/lib64/libpcre2-8.so /usr/lib64/libz.so /usr/lib64/liblzma.so /build/jenkins/workspace/lcg_nightly_pipeline/install/dev3/xxHash/0.8.3/x86_64-el9-gcc15-opt/lib/libxxhash.a /usr/lib64/liblz4.so /usr/lib64/libzstd.so
/cvmfs/sft.cern.ch/lcg/releases/binutils/2.40-acaab/x86_64-el9/bin/ld: /build/jenkins/workspace/lcg_nightly_pipeline/install/dev3/xxHash/0.8.3/x86_64-el9-gcc15-opt/lib/libxxhash.a(xxhash.o): relocation R_X86_64_32S against `.rodata' can not be used when making a shared object; recompile with -fPIC
/cvmfs/sft.cern.ch/lcg/releases/binutils/2.40-acaab/x86_64-el9/bin/ld: failed to set dynamic section sizes: bad value
collect2: error: ld returned 1 exit status

I left the Windows check since that is how it is done for the dynamic library.
Alternatively we could change CFLAGS, but given that it is already there for the dynamic library I made this PR.