add option to build with system sexp library

CMakeLists.txt

@@ -58,6 +58,8 @@ option(ENABLE_COVERAGE "Enable code coverage testing.")
 option(ENABLE_SANITIZERS "Enable ASan and other sanitizers.")
 option(ENABLE_FUZZERS "Enable fuzz targets.")
 option(DOWNLOAD_GTEST "Download Googletest" On)
+option(SYSTEM_LIBSEXP "Use system sexp library" OFF)
+
 # crypto components
 function(tristate_feature_auto NAME DESCRIPTION)
   set(${NAME} Auto CACHE STRING ${DESCRIPTION})
@@ -182,13 +184,18 @@ if (ENABLE_FUZZERS)
 endif()
 add_subdirectory(src/common)
 
-set(WITH_SEXP_CLI OFF)
-set(WITH_SEXP_TESTS OFF)
-set(CMAKE_INSTALL_DEFAULT_COMPONENT_NAME development)
-set(SAVE_BUILD_SHARED_LIBS ${BUILD_SHARED_LIBS})
-set(BUILD_SHARED_LIBS OFF)
-add_subdirectory(src/libsexp EXCLUDE_FROM_ALL)
-set(BUILD_SHARED_LIBS ${SAVE_BUILD_SHARED_LIBS})
+if (SYSTEM_LIBSEXP)
+  find_package(PkgConfig)
+  pkg_check_modules(REQUIRED sexp>=0.8.5)
+else(SYSTEM_LIBSEXP)
+  set(WITH_SEXP_CLI OFF)
+  set(WITH_SEXP_TESTS OFF)
+  set(CMAKE_INSTALL_DEFAULT_COMPONENT_NAME development)
+  set(SAVE_BUILD_SHARED_LIBS ${BUILD_SHARED_LIBS})
+  set(BUILD_SHARED_LIBS OFF)
+  add_subdirectory(src/libsexp EXCLUDE_FROM_ALL)
+  set(BUILD_SHARED_LIBS ${SAVE_BUILD_SHARED_LIBS})
+endif(SYSTEM_LIBSEXP)
 
 add_subdirectory(src/lib)
 add_subdirectory(src/rnp)