tweak fPIC cmake and doc

CMakeLists.txt

@@ -19,7 +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_POSITION_INDEPENDENT_CODE "Whether to build the finufft library with position independent code (-fPIC). This is only effective when FINUFFT_STATIC_LINKING is ON." ON)
+option(FINUFFT_POSITION_INDEPENDENT_CODE "Whether to build the finufft library with position independent code (-fPIC). This forced ON when FINUFFT_SHARED_LINKING is ON." ON)
 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)

docs/devnotes.rst

@@ -5,6 +5,8 @@ Developer notes
 
 * Developers needing to update/regenerate documentation in general, including our readthedocs website, see ``docs/README``. Developers changing MATLAB/octave interfaces or docs, also see ``matlab/README``. Please also see ``contributing.md`` for code style and git hook guidelines.
 
+* FINUFFT is by default built with position-independent code (``-fPIC`` compile flag), by both the Makefile and CMake. See CMake options to change this for the static library. Developers changing the FINUFFT source code should use ``static`` functions unless they need to export the symbol; this will prevent performance degradation when using ``-fPIC``.
+
 * To update the version number, this needs to be done by hand in the following places (we decided that a version-bump script is not worth the hassle):
 
   - ``CMakeLists.txt`` for cmake