Cpp: Link to threads library

As detailed in #3708, it is necessary to link against the (p)threads
library in order to be able to use std::call_once without producing
linker errors.

Since this function is frequently used in ANTLR's cpp version, this
commit ensures that the respective library is always linked against in
order to avoid this error, even if downstream users are not explicitly
linking against an appropriate threads library.

Fixes #3708

Signed-off-by: Robert Adam <dev@robert-adam.de>

Co-authored-by: Bryan Tan <Technius@users.noreply.github.com>

runtime/Cpp/cmake/antlr4-runtime.cmake.in

@@ -5,6 +5,9 @@ set(ANTLR_VERSION @ANTLR_VERSION@)
 set_and_check(ANTLR4_INCLUDE_DIR "@PACKAGE_ANTLR4_INCLUDE_DIR@")
 set_and_check(ANTLR4_LIB_DIR "@PACKAGE_ANTLR4_LIB_DIR@")
 
+include(CMakeFindDependencyMacro)
+find_dependency(Threads)
+
 include(${CMAKE_CURRENT_LIST_DIR}/@targets_export_name@.cmake)
 
 check_required_components(antlr)

runtime/Cpp/runtime/CMakeLists.txt

@@ -28,6 +28,13 @@ file(GLOB libantlrcpp_SRC
 add_library(antlr4_shared SHARED ${libantlrcpp_SRC})
 add_library(antlr4_static STATIC ${libantlrcpp_SRC})
 
+# Make sure to link against threads (pthreads) library in order to be able to
+# make use of std::call_once in the code without producing runtime errors
+# (see also https://github.com/antlr/antlr4/issues/3708 and/or https://stackoverflow.com/q/51584960).
+find_package(Threads REQUIRED)
+target_link_libraries(antlr4_shared Threads::Threads)
+target_link_libraries(antlr4_static Threads::Threads)
+
 if (ANTLR_BUILD_CPP_TESTS)
   include(FetchContent)