cmake: link against liblzma when found, for libunwind

Some versions of libunwind need liblzma. If liblzma is found,
we link against it unconditionally. If the installed libunwind
didn't need it, this is a nop.

Tested and fine tuned by iDunk

cmake/FindLibunwind.cmake

@@ -17,6 +17,22 @@ find_path(LIBUNWIND_INCLUDE_DIR libunwind.h
 )
 
 find_library(LIBUNWIND_LIBRARIES NAMES unwind )
+if(NOT LIBUNWIND_LIBRARIES STREQUAL "LIBUNWIND_LIBRARIES-NOTFOUND")
+  if (CMAKE_COMPILER_IS_GNUCC)
+    set(LIBUNWIND_LIBRARIES "gcc_eh;${LIBUNWIND_LIBRARIES}")
+  endif()
+endif()
+
+# some versions of libunwind need liblzma, and we don't use pkg-config
+# so we just look whether liblzma is installed, and add it if it is.
+# It might not be actually needed, but doesn't hurt if it is not.
+# We don't need any headers, just the lib, as it's privately needed.
+message(STATUS "looking for liblzma")
+find_library(LIBLZMA_LIBRARIES lzma )
+if(NOT LIBLZMA_LIBRARIES STREQUAL "LIBLZMA_LIBRARIES-NOTFOUND")
+  message(STATUS "liblzma found")
+  set(LIBUNWIND_LIBRARIES "${LIBUNWIND_LIBRARIES};${LIBLZMA_LIBRARIES}")
+endif()
 
 include(FindPackageHandleStandardArgs)
 find_package_handle_standard_args(Libunwind "Could not find libunwind" LIBUNWIND_INCLUDE_DIR LIBUNWIND_LIBRARIES)