add fpic option for static lib

CMakeLists.txt

@@ -19,6 +19,7 @@ option(FINUFFT_USE_OPENMP "Whether to use OpenMP for parallelization. If disable
 option(FINUFFT_USE_CPU "Whether to build the ordinary FINUFFT library (libfinufft)." ON)
 option(FINUFFT_USE_CUDA "Whether to build CUDA accelerated FINUFFT library (libcufinufft). This is completely independent of the main FINUFFT library" OFF)
 option(FINUFFT_STATIC_LINKING "If ON builds the static finufft library, if OFF build a shared finufft library." ON)
+option(FINUFFT_STATIC_LINKING_PIC "Whether to build the FINUFFT static lib with -fPIC flag" OFF)
 option(FINUFFT_BUILD_DEVEL "Whether to build development executables" OFF)
 option(FINUFFT_BUILD_EXAMPLES "Whether to build the FINUFFT examples" OFF)
 option(FINUFFT_BUILD_TESTS "Whether to build the FINUFFT tests" OFF)
@@ -242,6 +243,9 @@ function(set_finufft_options target)
   endif()
   target_link_libraries(${target} PRIVATE xsimd)
   target_link_libraries(${target} PRIVATE ${FINUFFT_FFTLIBS})
+  if(FINUFFT_STATIC_LINKING AND FINUFFT_STATIC_LINKING_PIC)
+    set_property(TARGET ${target} PROPERTY POSITION_INDEPENDENT_CODE ON)
+  endif()
 endfunction()
 
 if(FINUFFT_USE_CPU)