[CMake] Provide Findzstd module

[mgorny]

Summary

This module is used to find the system zstd library. The imported
targets intentionally use the same name as the generated zstd config
CMake file so these can be used interchangeably.

Based on earlier work of Petr Hosek. The support for pkg-config is
inspired by FindLibEdit.cmake.

[mgorny]

Changes from D134990:

• fixed variable names to be lowercase, to match what LLVM expects
• added support for pkg-config, to provide directory hints
• added support for finding the static library as well when find_library() returns the shared library

[MaskRay]

If zstdConfig.cmake from zstd itself is available, does this still work?

[mgorny]

In D135153#3834093, @MaskRay wrote:

If zstdConfig.cmake from zstd itself is available, does this still work?

No clue. Any chance you could test this, please?

[MaskRay]

It works. Checked with -DLLVM_TARGETS_TO_BUILD='X86' -DLLVM_ENABLE_ZSTD=FORCE_ON -DCMAKE_PREFIX_PATH=/tmp/opt/zstd

[phosek]

This isn't going to work on Windows which uses a different naming scheme, see also D134990#3834272.

[mgorny]

In D135153#3834528, @phosek wrote:

This isn't going to work on Windows which uses a different naming scheme, see also D134990#3834272.

Is this actually a regression, i.e. does the existing CMake support stop working on Windows?

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, the following import targets will be defined:
+# zstd::libzstd_shared
+# zstd::libzstd_static
+
+find_package(PkgConfig QUIET)
+pkg_check_modules(PC_LIBZSTD QUIET libzstd)
+
+find_path(zstd_INCLUDE_DIR NAMES zstd.h HINTS ${PC_LIBZSTD_INCLUDE_DIRS})
+find_library(zstd_LIBRARY NAMES zstd HINTS ${PC_LIBZSTD_LIBRARY_DIRS})
+
+include(FindPackageHandleStandardArgs)
+find_package_handle_standard_args(
+  zstd DEFAULT_MSG
+  zstd_LIBRARY zstd_INCLUDE_DIR)
+
+if(zstd_FOUND)
+  if(zstd_LIBRARY MATCHES "\\${CMAKE_SHARED_LIBRARY_SUFFIX}$" AND
+     NOT TARGET zstd::libzstd_shared)
+    add_library(zstd::libzstd_shared UNKNOWN IMPORTED)
+    set_target_properties(zstd::libzstd_shared PROPERTIES
+      INTERFACE_INCLUDE_DIRECTORIES "${zstd_INCLUDE_DIR}"
+      IMPORTED_LOCATION "${zstd_LIBRARY}")
+
+    # Try to find the static library in addition to the shared library.
+    find_library(zstd_STATIC_LIBRARY
+      NAMES "${CMAKE_STATIC_LIBRARY_PREFIX}zstd${CMAKE_STATIC_LIBRARY_SUFFIX}")
+  else()
+    set(zstd_STATIC_LIBRARY "${zstd_LIBRARY}")
+  endif()
+  if(zstd_STATIC_LIBRARY MATCHES "\\${CMAKE_STATIC_LIBRARY_SUFFIX}$" AND
+     NOT TARGET zstd::libzstd_static)
+    add_library(zstd::libzstd_static UNKNOWN IMPORTED)
+    set_target_properties(zstd::libzstd_static PROPERTIES
+      INTERFACE_INCLUDE_DIRECTORIES "${zstd_INCLUDE_DIR}"
+      IMPORTED_LOCATION "${zstd_STATIC_LIBRARY}")
+  endif()
+endif()
+
+mark_as_advanced(zstd_INCLUDE_DIR zstd_LIBRARY zstd_STATIC_LIBRARY)