[CMake] Use NOT instead of STREQUAL

[phosek]

Summary

<var> STREQUAL "" fails when <var> is unset which can be the
case when using runtimes as top-level build. Use NOT instead.

[mstorsjo]

LGTM, and sorry for the sloppy testing of the reland of this :-(

[phosek]

In D109570#2993640, @mstorsjo wrote:

LGTM, and sorry for the sloppy testing of the reland of this :-(

No worries and thanks for the quick review ;) The number of supported configuration makes it difficult to test everything. I hope it's something we can address over time.

runtimes/CMakeLists.txt

if(compiler_rt_path)
   if(NOT DEFINED LLVM_BUILD_COMPILER_RT OR LLVM_BUILD_COMPILER_RT)
     list(INSERT runtimes 0 ${compiler_rt_path})
   endif()
 endif()
 
 # If building standalone by pointing CMake at this runtimes directory,
 # LLVM_BINARY_DIR isn't set, find_package(LLVM) will fail and these
 # intermediate paths are unset.
-if (LLVM_BINARY_DIR STREQUAL "")
+if (NOT LLVM_BINARY_DIR)
   set(LLVM_BINARY_DIR ${CMAKE_CURRENT_BINARY_DIR})
 endif()
 if (NOT LLVM_FOUND)
   set(LLVM_TOOLS_BINARY_DIR ${LLVM_BINARY_DIR}/bin)
   set(LLVM_LIBRARY_DIR ${CMAKE_CURRENT_BINARY_DIR}/lib)
 endif()
 
 # Setting these variables will allow the sub-build to put their outputs into