[libunwind] Provide a way to set '_LIBUNWIND_IS_BAREMETAL' through cmake.

[abidh]

Summary

Libunwind uses _LIBUNWIND_IS_BAREMETAL in a lot of places but there is no cmake variable to set it. This patch adds such a variable. It is quite like what LIBCXXABI_BAREMETAL does in libcxxabi.

[compnerd]

Minor nits, but this seems like a reasonable control to give to the user.

libunwind/CMakeLists.txt

365

LLVM style is to not have the space after the if.

366

Could you use add_compile_definitions instead please?

[abidh]

libunwind/CMakeLists.txt

365

Done. But I noticed that most if in this file have a space after them.

[abidh]

Handled review comments.

libunwind/CMakeLists.txt

 option(LIBUNWIND_ENABLE_SHARED "Build libunwind as a shared library." ON)
 option(LIBUNWIND_ENABLE_STATIC "Build libunwind as a static library." ON)
 option(LIBUNWIND_ENABLE_CROSS_UNWINDING "Enable cross-platform unwinding support." OFF)
 option(LIBUNWIND_ENABLE_ARM_WMMX "Enable unwinding support for ARM WMMX registers." OFF)
 option(LIBUNWIND_ENABLE_THREADS "Build libunwind with threading support." ON)
 option(LIBUNWIND_WEAK_PTHREAD_LIB "Use weak references to refer to pthread functions." OFF)
 option(LIBUNWIND_USE_COMPILER_RT "Use compiler-rt instead of libgcc" OFF)
 option(LIBUNWIND_INCLUDE_DOCS "Build the libunwind documentation." ${LLVM_INCLUDE_DOCS})
+option(LIBUNWIND_IS_BAREMETAL "Build libunwind for baremetal targets." OFF)
 
 set(LIBUNWIND_LIBDIR_SUFFIX "${LLVM_LIBDIR_SUFFIX}" CACHE STRING
     "Define suffix of library directory name (32/64)")
 option(LIBUNWIND_INSTALL_LIBRARY "Install the libunwind library." ON)
 cmake_dependent_option(LIBUNWIND_INSTALL_STATIC_LIBRARY
   "Install the static libunwind library." ON
   "LIBUNWIND_ENABLE_STATIC;LIBUNWIND_INSTALL_LIBRARY" OFF)
 cmake_dependent_option(LIBUNWIND_INSTALL_SHARED_LIBRARY

 if (LIBUNWIND_ENABLE_ARM_WMMX)
   # __ARM_WMMX is a compiler pre-define (as per the ACLE 2.0). Clang does not
   # define this macro for any supported target at present. Therefore, here we
   # provide the option to explicitly enable support for WMMX registers in the
   # unwinder.
   add_compile_flags(-D__ARM_WMMX)
 endif()
 
+if(LIBUNWIND_IS_BAREMETAL)
+  add_compile_definitions(_LIBUNWIND_IS_BAREMETAL)
+endif()
+
 # This is the _ONLY_ place where add_definitions is called.
 if (MSVC)
   add_definitions(-D_CRT_SECURE_NO_WARNINGS)
 endif()
 
 # Disable DLL annotations on Windows for static builds.
 if (WIN32 AND LIBUNWIND_ENABLE_STATIC AND NOT LIBUNWIND_ENABLE_SHARED)
   add_definitions(-D_LIBUNWIND_DISABLE_VISIBILITY_ANNOTATIONS)