CMake: Properly include zstd include directories

CMake/DolphinLibraryTools.cmake

@@ -0,0 +1,10 @@
+# like add_library(new ALIAS old) but avoids add_library cannot create ALIAS target "new" because target "old" is imported but not globally visible. on older cmake
+# This can be replaced with a direct alias call once our minimum is cmake 3.18
+function(dolphin_alias_library new old)
+  string(REPLACE "::" "" library_no_namespace ${old})
+  if (NOT TARGET _alias_${library_no_namespace})
+    add_library(_alias_${library_no_namespace} INTERFACE)
+    target_link_libraries(_alias_${library_no_namespace} INTERFACE ${old})
+  endif()
+  add_library(${new} ALIAS _alias_${library_no_namespace})
+endfunction()

CMakeLists.txt

@@ -127,6 +127,7 @@ include(CheckCCompilerFlag)
 include(CheckVendoringApproved)
 include(DolphinCompileDefinitions)
 include(DolphinDisableWarningsMSVC)
+include(DolphinLibraryTools)
 include(RemoveCompileFlag)
 
 # Enable folders for IDE
@@ -773,17 +774,13 @@ else()
   add_subdirectory(Externals/liblzma)
 endif()
 
-pkg_check_modules(ZSTD QUIET libzstd>=1.4.0)
-check_include_file(zstd.h HAVE_ZSTD_H)
-if(ZSTD_FOUND AND HAVE_ZSTD_H)
+pkg_check_modules(ZSTD QUIET libzstd>=1.4.0 IMPORTED_TARGET)
+if(ZSTD_FOUND)
   message(STATUS "Using shared zstd version: " ${ZSTD_VERSION})
+  dolphin_alias_library(zstd::zstd PkgConfig::ZSTD)
 else()
   check_vendoring_approved(zstd)
-  if(ZSTD_FOUND AND NOT HAVE_ZSTD_H)
-    message(STATUS "Shared zstd found but lacks headers, falling back to the static library")
-  else()
-    message(STATUS "Shared zstd not found, falling back to the static library")
-  endif()
+  message(STATUS "Shared zstd not found, falling back to the static library")
   add_subdirectory(Externals/zstd)
 endif()
 

Source/Core/DiscIO/CMakeLists.txt

@@ -71,7 +71,7 @@ PUBLIC
   core
   BZip2::BZip2
   lzma
-  zstd
+  zstd::zstd
 
 PRIVATE
   fmt::fmt