[llvm] [cmake] Support finding both static and shared zstd via FindZstd

[mgorny]

Summary

Improve the logic in FindZstd to support finding both shared and static
variants of the zstd library simultaneously. Otherwise, if the shared
library is installed, zstd::libzstd_static is not declared at all
and CMake fails if LLVM_USE_STATIC_ZSTD is used.

[mgorny]

Thanks!

[phosek]

Alternative that I'd slightly prefer would be to only search for static library when LLVM_USE_STATIC_ZSTD is enabled, otherwise try both as before. That's more in line with what the other CMake modules do. I haven't found any module that would try to find both shared and static library at the same time and I'm not sure if there's a use case for it.

[mgorny]

However, exporting both targets is in line with what zstd cmake configs do.

llvm/cmake/modules/Findzstd.cmake

 # Try to find the zstd library
 #
 # If successful, the following variables will be defined:
 # zstd_INCLUDE_DIR
 # zstd_LIBRARY
+# zstd_STATIC_LIBRARY
 # zstd_FOUND
 #
 # Additionally, one of the following import targets will be defined:
 # zstd::libzstd_shared
 # zstd::libzstd_static
 
 if(MSVC)
   set(zstd_SHARED_LIBRARY_SUFFIX "\\${CMAKE_LINK_LIBRARY_SUFFIX}$")
   set(zstd_STATIC_LIBRARY_SUFFIX "_static\\${CMAKE_STATIC_LIBRARY_SUFFIX}$")
 else()
   set(zstd_SHARED_LIBRARY_SUFFIX "\\${CMAKE_SHARED_LIBRARY_SUFFIX}$")
   set(zstd_STATIC_LIBRARY_SUFFIX "\\${CMAKE_STATIC_LIBRARY_SUFFIX}$")
 endif()
 
 find_path(zstd_INCLUDE_DIR NAMES zstd.h)
 find_library(zstd_LIBRARY NAMES zstd zstd_static)
+find_library(zstd_STATIC_LIBRARY NAMES
+  zstd_static
+  "${CMAKE_STATIC_LIBRARY_PREFIX}zstd${CMAKE_STATIC_LIBRARY_SUFFIX}")
 
 include(FindPackageHandleStandardArgs)
 find_package_handle_standard_args(
     zstd DEFAULT_MSG
     zstd_LIBRARY zstd_INCLUDE_DIR
 )
 
 if(zstd_FOUND)
   if(zstd_LIBRARY MATCHES "${zstd_SHARED_LIBRARY_SUFFIX}$" AND
      NOT TARGET zstd::libzstd_shared)
     add_library(zstd::libzstd_shared SHARED IMPORTED)
     set_target_properties(zstd::libzstd_shared PROPERTIES
           INTERFACE_INCLUDE_DIRECTORIES "${zstd_INCLUDE_DIR}"
           IMPORTED_LOCATION "${zstd_LIBRARY}")
+  else()
+    set(zstd_STATIC_LIBRARY "${zstd_LIBRARY}")
   endif()
-  if(zstd_LIBRARY MATCHES "${zstd_STATIC_LIBRARY_SUFFIX}$" AND
+  if(zstd_STATIC_LIBRARY MATCHES "${zstd_STATIC_LIBRARY_SUFFIX}$" AND
      NOT TARGET zstd::libzstd_static)
     add_library(zstd::libzstd_static STATIC IMPORTED)
     set_target_properties(zstd::libzstd_static PROPERTIES
         INTERFACE_INCLUDE_DIRECTORIES "${zstd_INCLUDE_DIR}"
-        IMPORTED_LOCATION "${zstd_LIBRARY}")
+        IMPORTED_LOCATION "${zstd_STATIC_LIBRARY}")
   endif()
 endif()
 
 unset(zstd_SHARED_LIBRARY_SUFFIX)
 unset(zstd_STATIC_LIBRARY_SUFFIX)
 
-mark_as_advanced(zstd_INCLUDE_DIR zstd_LIBRARY)
+mark_as_advanced(zstd_INCLUDE_DIR zstd_LIBRARY zstd_STATIC_LIBRARY)