[llvm-config] Make llvm-config --system-libs obey LLVM_USE_STATIC_ZSTD.

LLVM's build system does the right thing but LLVM_SYSTEM_LIBS ends up
containing the shared library. Emit the static library instead when
appropriate.

With LLVM_USE_STATIC_ZSTD, before:

khuey@zhadum:~/dev/llvm-project/build$ ./bin/llvm-config --system-libs
-lrt -ldl -lm -lz -lzstd -lxml2

after:

khuey@zhadum:~/dev/llvm-project/build$ ./bin/llvm-config --system-libs
-lrt -ldl -lm -lz /usr/local/lib/libzstd.a -lxml2

Author: khuey

llvm/lib/Support/CMakeLists.txt

@@ -323,8 +323,12 @@ if(LLVM_ENABLE_ZSTD)
   if(NOT zstd_library)
     get_property(zstd_library TARGET ${zstd_target} PROPERTY LOCATION)
   endif()
-  get_library_name(${zstd_library} zstd_library)
-  set(llvm_system_libs ${llvm_system_libs} "${zstd_library}")
+  if (zstd_target STREQUAL zstd::libzstd_shared)
+    get_library_name(${zstd_library} zstd_library)
+    set(llvm_system_libs ${llvm_system_libs} "${zstd_library}")
+  else()
+    set(llvm_system_libs ${llvm_system_libs} "${zstd_STATIC_LIBRARY}")
+  endif()
 endif()
 
 set_property(TARGET LLVMSupport PROPERTY LLVM_SYSTEM_LIBS "${llvm_system_libs}")