[CMake] linking static libs into shared libs on Linux

[Steve Borho]

Hello, I am the technical lead for x265 (HEVC encoder) and we have used
cmake successfully since the beginning of the project.  There are a few
rough edges that we've had to work around (uninstall rules, yasm support
in MSVC, etc) but in general it has worked pretty well.

x265 can support a number of bit-depth options, but these are compile
time decisions. You must chose at build time whether the encoder will
use 8bit, 10bit, or 12bit color samples internally.  Recently we've
resolved the naming conflicts and thus allow these multiple builds of
libx265 to be linked together into a single binary.

The way the multilib build process currently works is it creates 8bit/
10bit/ and 12bit/ folders, then runs cmake in each seperately (with
different options) so there are three distinct build folders. It builds
10bit and 12bit first, outputting only static libraries, then finally it
builds the 8bit folder and adds the 10bit and 12bit static libraries to
the 8bit shared library link target.

This works well on Windows and Mac but on x86_64 Linux I get this error:

/bin/ld: common/CMakeFiles/common.dir/x86/pixel-a.asm.o: relocation
R_X86_64_PC32 against symbol `x265_pw_1' can not be used when making a
shared object; recompile with -fPIC

(in a nutshell, the 10bit and 12bit static libs are no longer position
independent)

I figure there are multiple potential solutions for this and would
welcome pointers for any of them

1 - is there a way for the project to re-include itself? If so we could
  have the 8bit build simply build the 10 and 12bit object libraries and
  link with those directly and bypass the creation of static libraries.
  This would perhaps be optimal since you would end up with a single
  Makefile driving the whole build.

2 - is there some invocation that will force cmake to build the static
  libraries such that they are still safe for linking into a shared
  library?

  I've already tried these unsuccessfully:
    set(CMAKE_POSITION_INDEPENDENT_CODE TRUE)
    set_target_properties(x265-static PROPERTIES POSITION_INDEPENDENT_CODE TRUE)
    set_target_properties(x265-static PROPERTIES LINK_FLAGS "-fPIC")

3 - is there another intermediate format that cmake can output that
  preserves -fPIC, or some way to access the object libraries of the
  other external builds?

4 - is there any post-processing that can be done on the static libs to
  make them safe for shared libs again (guessing this is not likely)

(note: I'm building on Centos 7, but this does not appear to be
distribution dependent, and I've tried cmake 2.8.11 and 3.2.3)

Thanks

-- 
Steve Borho

PS: Our source code is at https://bitbucket.org/multicoreware/x265