allow one to use system ZSTD

CMakeLists.txt

@@ -7,6 +7,7 @@ if(CMAKE_PROJECT_NAME STREQUAL "chdr")
 endif()
 option(INSTALL_STATIC_LIBS "Install static libraries" OFF)
 option(WITH_SYSTEM_ZLIB "Use system provided zlib library" OFF)
+option(WITH_SYSTEM_ZSTD "Use system provided zstd library" OFF)
 
 option(BUILD_LTO "Compile libchdr with link-time optimization if supported" OFF)
 if(BUILD_LTO)
@@ -41,11 +42,17 @@ else()
 endif()
 
 # zstd
-option(ZSTD_BUILD_SHARED "BUILD SHARED LIBRARIES" OFF)
-option(ZSTD_BUILD_PROGRAMS "BUILD PROGRAMS" OFF)
-option(ZSTD_LEGACY_SUPPORT "LEGACY SUPPORT" OFF)
-add_subdirectory(deps/zstd-1.5.6/build/cmake EXCLUDE_FROM_ALL)
-list(APPEND CHDR_LIBS libzstd_static)
+if (WITH_SYSTEM_ZSTD)
+  find_path(ZSTD_INCLUDE_DIR NAMES zstd.h)
+  find_library(ZSTD_LIBRARIES NAMES zstd HINTS ${ZSTD_ROOT_DIR}/lib)
+  list(APPEND CHDR_LIBS zstd)
+else()
+  option(ZSTD_BUILD_SHARED "BUILD SHARED LIBRARIES" OFF)
+  option(ZSTD_BUILD_PROGRAMS "BUILD PROGRAMS" OFF)
+  option(ZSTD_LEGACY_SUPPORT "LEGACY SUPPORT" OFF)
+  add_subdirectory(deps/zstd-1.5.6/build/cmake EXCLUDE_FROM_ALL)
+  list(APPEND CHDR_LIBS libzstd_static)
+endif()
 #--------------------------------------------------
 # chdr
 #--------------------------------------------------