build(cmake): improve pkg-config generation

With this patch the pkg-config generation when using the CMake build
system is improved in the following ways:

- Libs.private is now filled when needed
- The JoinPaths module is now used to join paths, leading to simpler
  code
- The .pc file is always generated, regardless of the platform, as it
  can also be consumed on Windows

Here's how the .pc file is affected by these changes, in comparison to
the one generated with the official Makefiles:

    $ diff -s lib/libzstd.pc build/cmake/build-old/lib/libzstd.pc
    15c15
    < Libs.private: -pthread
    ---
    > Libs.private:

    $ diff -s lib/libzstd.pc build/cmake/build-new/lib/libzstd.pc
    Files lib/libzstd.pc and build/cmake/build-new/lib/libzstd.pc are
    identical

build/cmake/CMakeModules/JoinPaths.cmake

@@ -0,0 +1,23 @@
+# This module provides function for joining paths
+# known from most languages
+#
+# SPDX-License-Identifier: (MIT OR CC0-1.0)
+# Copyright 2020 Jan Tojnar
+# https://github.com/jtojnar/cmake-snips
+#
+# Modelled after Python’s os.path.join
+# https://docs.python.org/3.7/library/os.path.html#os.path.join
+# Windows not supported
+function(join_paths joined_path first_path_segment)
+    set(temp_path "${first_path_segment}")
+    foreach(current_segment IN LISTS ARGN)
+        if(NOT ("${current_segment}" STREQUAL ""))
+            if(IS_ABSOLUTE "${current_segment}")
+                set(temp_path "${current_segment}")
+            else()
+                set(temp_path "${temp_path}/${current_segment}")
+            endif()
+        endif()
+    endforeach()
+    set(${joined_path} "${temp_path}" PARENT_SCOPE)
+endfunction()

build/cmake/lib/CMakeLists.txt

@@ -135,30 +135,17 @@ if (ZSTD_BUILD_STATIC)
             OUTPUT_NAME ${STATIC_LIBRARY_BASE_NAME})
 endif ()
 
-if (UNIX OR MINGW)
+# pkg-config
-    # pkg-config
+include(JoinPaths) # can be replaced by cmake_path(APPEND) in CMake 3.20
-    set(PREFIX "${CMAKE_INSTALL_PREFIX}")
+set(PREFIX "${CMAKE_INSTALL_PREFIX}")
-    set(EXEC_PREFIX "\${prefix}")
+set(EXEC_PREFIX "\${prefix}")
-    set(LIBDIR "${CMAKE_INSTALL_FULL_LIBDIR}")
+join_paths(LIBDIR "\${exec_prefix}" "${CMAKE_INSTALL_LIBDIR}")
-    set(INCLUDEDIR "${CMAKE_INSTALL_FULL_INCLUDEDIR}")
+join_paths(INCLUDEDIR "\${prefix}" "${CMAKE_INSTALL_INCLUDEDIR}")
-    set(VERSION "${zstd_VERSION}")
+set(LIBS_PRIVATE "${THREADS_LIBS}")
+set(VERSION "${zstd_VERSION}")
 
-    string(LENGTH "${PREFIX}" PREFIX_LENGTH)
+configure_file("${LIBRARY_DIR}/libzstd.pc.in" "${CMAKE_CURRENT_BINARY_DIR}/libzstd.pc" @ONLY)
-    string(SUBSTRING "${LIBDIR}" 0 ${PREFIX_LENGTH} LIBDIR_PREFIX)
+install(FILES "${CMAKE_CURRENT_BINARY_DIR}/libzstd.pc" DESTINATION "${CMAKE_INSTALL_LIBDIR}/pkgconfig")
-    string(SUBSTRING "${LIBDIR}" ${PREFIX_LENGTH} -1 LIBDIR_SUFFIX)
-    string(SUBSTRING "${INCLUDEDIR}" 0 ${PREFIX_LENGTH} INCLUDEDIR_PREFIX)
-    string(SUBSTRING "${INCLUDEDIR}" ${PREFIX_LENGTH} -1 INCLUDEDIR_SUFFIX)
-
-    if ("${INCLUDEDIR_PREFIX}" STREQUAL "${PREFIX}")
-        set(INCLUDEDIR "\${prefix}${INCLUDEDIR_SUFFIX}")
-    endif()
-    if ("${LIBDIR_PREFIX}" STREQUAL "${PREFIX}")
-        set(LIBDIR "\${exec_prefix}${LIBDIR_SUFFIX}")
-    endif()
-
-    configure_file("${LIBRARY_DIR}/libzstd.pc.in" "${CMAKE_CURRENT_BINARY_DIR}/libzstd.pc" @ONLY)
-    install(FILES "${CMAKE_CURRENT_BINARY_DIR}/libzstd.pc" DESTINATION "${CMAKE_INSTALL_LIBDIR}/pkgconfig")
-endif ()
 
 # install target
 install(FILES