This fixes a bug on x86_64-linux-gnu whereby libnlopt.a would contain
non-PIC code, leading to a link failure when building shared objects:

  g++ -o sources/build/libnonlinear_fitter_nlopt.so -shared sources/plugins/nonlinear_fitter_nlopt/fitter.os -Lexternal/build/lib -Lsources/build -Lexternal/build/lib -ldl -lrt -lnlopt -lm -lcore
  ld: external/build/lib/libnlopt.a(optimize.o): relocation R_X86_64_32S against `.text' can not be used when making a shared object; recompile with -fPIC
  external/build/lib/libnlopt.a: error adding symbols: Bad value
  collect2: error: ld returned 1 exit status