build: add benchmark compatibility with older glibc versions

Author: Raimo33

src/CMakeLists.txt

@@ -125,12 +125,17 @@ endif()
 unset(${PROJECT_NAME}_soversion)
 
 if(SECP256K1_BUILD_BENCHMARK)
+  find_library(RT_LIBRARY rt)
+  add_library(optional_rt INTERFACE)
+  if(RT_LIBRARY)
+    target_link_libraries(optional_rt INTERFACE rt)
+  endif()
   add_executable(secp256k1_bench bench.c)
   add_executable(secp256k1_bench_internal bench_internal.c)
   add_executable(secp256k1_bench_ecmult bench_ecmult.c)
-  target_link_libraries(secp256k1_bench secp256k1)
-  target_link_libraries(secp256k1_bench_internal secp256k1_precomputed secp256k1_asm)
-  target_link_libraries(secp256k1_bench_ecmult secp256k1_precomputed secp256k1_asm)
+  target_link_libraries(secp256k1_bench secp256k1 optional_rt)
+  target_link_libraries(secp256k1_bench_internal secp256k1_precomputed secp256k1_asm optional_rt)
+  target_link_libraries(secp256k1_bench_ecmult secp256k1_precomputed secp256k1_asm optional_rt)
 endif()
 
 if(SECP256K1_BUILD_TESTS)