[libc++] Move cache variable definition. NFC Move it to where the other ABI cache variables/options are defined. llvm-svn: 314950

diff --git a/libcxx/CMakeLists.txt b/libcxx/CMakeLists.txt
index ccf4b96..ea323af 100644
--- a/libcxx/CMakeLists.txt
+++ b/libcxx/CMakeLists.txt

@@ -101,6 +101,7 @@
 option(LIBCXX_ABI_UNSTABLE "Unstable ABI of libc++." OFF)
 option(LIBCXX_ABI_ITANIUM "Ignore auto-detection and force use of the Itanium ABI.")
 option(LIBCXX_ABI_MICROSOFT "Ignore auto-detection and force use of the Microsoft ABI.")
+set(LIBCXX_ABI_DEFINES "" CACHE STRING "A semicolon separated list of ABI macros to define in the site config header.")
 option(LIBCXX_USE_COMPILER_RT "Use compiler-rt instead of libgcc" OFF)
 
 if (NOT LIBCXX_ENABLE_SHARED AND NOT LIBCXX_ENABLE_STATIC)
@@ -615,7 +616,6 @@
 config_define_if(LIBCXX_BUILD_EXTERNAL_THREAD_LIBRARY _LIBCPP_HAS_THREAD_LIBRARY_EXTERNAL)
 config_define_if(LIBCXX_HAS_MUSL_LIBC _LIBCPP_HAS_MUSL_LIBC)
 
-set(LIBCXX_ABI_DEFINES "" CACHE STRING "A semicolon separated list of ABI macros to define in the site config header")
 if (LIBCXX_ABI_DEFINES)
   set(abi_defines)
   foreach (abi_define ${LIBCXX_ABI_DEFINES})