Workaround lack of -fPIC when building static libcxxabi

Author: ldionne

libcxx/cmake/Modules/HandleLibCXXABI.cmake

@@ -168,11 +168,17 @@ function(setup_abi_library abi_target linked_into input)
   # Link against the in-tree libc++abi.
   elseif ("${stdlib}" STREQUAL "libcxxabi")
     if (${merged})
+      if ("${linked_into}" STREQUAL "STATIC")
+        set(merge_target "cxxabi_static")
+      else()
+        set(merge_target "cxxabi_shared_for_merging")
+      endif()
+      message(STATUS "LDIONNE: merge_target=${merge_target}")
       add_library(${abi_target} INTERFACE)
       _merge_static_library(${abi_target}
-        "$<TARGET_PROPERTY:cxxabi_static,LIBRARY_OUTPUT_DIRECTORY>/${CMAKE_STATIC_LIBRARY_PREFIX}$<TARGET_PROPERTY:cxxabi_static,OUTPUT_NAME>${CMAKE_STATIC_LIBRARY_SUFFIX}")
+        "$<TARGET_PROPERTY:${merge_target},LIBRARY_OUTPUT_DIRECTORY>/${CMAKE_STATIC_LIBRARY_PREFIX}$<TARGET_PROPERTY:${merge_target},OUTPUT_NAME>${CMAKE_STATIC_LIBRARY_SUFFIX}")
       target_link_libraries(${abi_target} INTERFACE cxxabi-headers)
-      add_dependencies(${abi_target} cxxabi_static)
+      add_dependencies(${abi_target} ${merge_target})
     else()
       string(TOLOWER "${search_type}" type)
       add_library(${abi_target} INTERFACE)

libcxxabi/src/CMakeLists.txt

@@ -184,6 +184,21 @@ if (CMAKE_POSITION_INDEPENDENT_CODE OR NOT DEFINED CMAKE_POSITION_INDEPENDENT_CO
 endif()
 target_compile_options(cxxabi_shared_objects PRIVATE "${LIBCXXABI_ADDITIONAL_COMPILE_FLAGS}")
 
+add_library(cxxabi_shared_for_merging STATIC EXCLUDE_FROM_ALL)
+target_link_libraries(cxxabi_shared_for_merging
+  PUBLIC cxxabi_shared_objects
+  PRIVATE ${LIBCXXABI_LIBRARIES})
+set_target_properties(cxxabi_shared_for_merging
+  PROPERTIES
+    OUTPUT_NAME "${LIBCXXABI_STATIC_OUTPUT_NAME}-for-merging"
+  )
+if (LIBCXXABI_USE_LLVM_UNWINDER)
+  if (LIBCXXABI_STATICALLY_LINK_UNWINDER_IN_SHARED_LIBRARY OR
+      (DEFINED LIBUNWIND_ENABLE_SHARED AND NOT LIBUNWIND_ENABLE_SHARED))
+    target_sources(cxxabi_shared_for_merging PUBLIC $<TARGET_OBJECTS:unwind_shared_objects>)
+  endif()
+endif()
+
 add_library(cxxabi_shared SHARED)
 set_target_properties(cxxabi_shared
   PROPERTIES