Allow explicit pthread opt-in

When building libcxxabi in tree (i.e. in llvm/projects/libcxxabi,
along with llvm/projects/libcxx), libcxx's config_site.in doesn't
get created in a timely manner. This means that any configuration
that is normally set in libcxx's config_site.in needs to be
duplicated in libcxxabi to successfully build libcxxabi. This patch
does exactly that for the _LIBCPP_HAS_THREAD_API_PTHREAD
preprocessor define.

http://reviews.llvm.org/D20574


git-svn-id: https://llvm.org/svn/llvm-project/libcxxabi/trunk@270732 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 50d1fd7..37cc560 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -113,6 +113,7 @@
 option(LIBCXXABI_ENABLE_WERROR "Fail and stop if a warning is triggered." OFF)
 option(LIBCXXABI_USE_LLVM_UNWINDER "Build and use the LLVM unwinder." OFF)
 option(LIBCXXABI_ENABLE_THREADS "Build with threads enabled" ON)
+option(LIBCXXABI_HAS_PTHREAD_API "Ignore auto-detection and force use of pthread API" OFF)
 set(LIBCXXABI_GCC_TOOLCHAIN "" CACHE STRING "GCC toolchain for cross compiling.")
 set(LIBCXXABI_SYSROOT "" CACHE STRING "Sysroot for cross compiling.")
 set(LIBCXXABI_LIBCXX_LIBRARY_PATH "" CACHE STRING "The path to libc++ library.")
@@ -272,9 +273,18 @@
 endif()
 
 if (NOT LIBCXXABI_ENABLE_THREADS)
+  if (LIBCXXABI_HAS_PTHREAD_API)
+    message(FATAL_ERROR "LIBCXXABI_HAS_PTHREAD_API can only"
+                        " be set to ON when LIBCXXABI_ENABLE_THREADS"
+                        " is also set to ON.")
+  endif()
   add_definitions(-DLIBCXXABI_HAS_NO_THREADS=1)
 endif()
 
+if (LIBCXXABI_HAS_PTHREAD_API)
+  add_definitions(-D_LIBCPP_HAS_THREAD_API_PTHREAD)
+endif()
+
 # This is the _ONLY_ place where add_definitions is called.
 if (MSVC)
   add_definitions(-D_CRT_SECURE_NO_WARNINGS)