Build: Use PIC for jsimd_none.o in shared libs

In theory, all objects that will be included in a Un*x shared library
must be built using PIC.  In practice, most compilers don't require PIC
to be explicitly specified for jsimd_none.o, either because the compiler
automatically enables PIC in all cases (Ubuntu) or because the size of
the generated object is too small.  But some rare compilers do require
PIC to be explicitly specified for jsimd_none.o.

Fixes #520

CMakeLists.txt

@@ -582,6 +582,9 @@ if(WITH_SIMD)
   endif()
 else()
   add_library(simd OBJECT jsimd_none.c)
+  if(NOT WIN32 AND (CMAKE_POSITION_INDEPENDENT_CODE OR ENABLE_SHARED))
+    set_target_properties(simd PROPERTIES POSITION_INDEPENDENT_CODE 1)
+  endif()
 endif()
 
 if(WITH_JAVA)