[CMake] don't build libLTO when LLVM_ENABLE_PIC is OFF

[sugak]

Summary

When cmake is run with -DLLVM_ENABLE_PIC=OFF, build fails while
linking shared library libLTO.so, because its dependencies are built
with -fno-PIC. More details here: https://llvm.org/bugs/show_bug.cgi?id=26484.
This diff reverts r252652 (git 9fd4377ddb83aee3c049dc8757e7771edbb8ee71),
which removed check NOT LLVM_ENABLE_PIC before disabling build for libLTO.so.

I've verified that when cmake is run with -DLLVM_ENABLE_PIC=ON, libLTO.so is
still generated, as well as when this option is not specified and the default
value (ON) is used. Tested both on master and release_38.

[mehdi_amini]

That makes sense to me, but I'm not sure what Chris meant when he wrote: "Autoconf builds libLTO with -fPIC, CMake should be able to as well", it seems his change in r252652 enabled building libLTO *without* PIC?

[sugak]

@joker.eph: yes, in environments that don't set CYGWIN, after r252652 libLTO is always built, regardless of LLVM_ENABLE_PIC.

[hans]

This seems reasonable to me too, but it would be good to hear what Chris thinks.

[beanz]

The LLVM_ENABLE_PIC option is oddly implemented on Darwin, and is really just broken. I have on my todo list to make better sense of it on Darwin.

This patch looks good, and should land.

Thanks,
-Chris

[sugak]

I need someone to commit this for me.

Thank you!

[hans]

And merged to 3.8 in r260704.

tools/CMakeLists.txt

 if(NOT LLVM_BUILD_LLVM_DYLIB )
   set(LLVM_TOOL_LLVM_SHLIB_BUILD Off)
 endif()
 
 if(NOT LLVM_USE_INTEL_JITEVENTS )
   set(LLVM_TOOL_LLVM_JITLISTENER_BUILD Off)
 endif()
 
-if(CYGWIN)
+if(CYGWIN OR NOT LLVM_ENABLE_PIC)
   set(LLVM_TOOL_LTO_BUILD Off)
   set(LLVM_TOOL_LLVM_LTO_BUILD Off)
 endif()
 
 # Add LTO, llvm-ar, llvm-config, and llvm-profdata before clang, ExternalProject
 # requires targets specified in DEPENDS to exist before the call to
 # ExternalProject_Add.
 add_llvm_tool_subdirectory(lto)