Merge #1600: cmake: Introduce SECP256K1_APPEND_LDFLAGS variable

421ed1b cmake: Introduce `SECP256K1_APPEND_LDFLAGS` variable (Hennadii Stepanov)

Pull request description:

ACKs for top commit:
  jonasnick:
    ACK 421ed1b

Tree-SHA512: 044b84b86a98e30268189dc92c4b8045e146451f3849a9863eb8f99490df53f1f02ae0fc74d74dc6ede0ea0c55653c142f409c77e1967505a085f31e4c472443

CMakeLists.txt

@@ -276,6 +276,14 @@ if(SECP256K1_APPEND_CFLAGS)
   string(APPEND CMAKE_C_COMPILE_OBJECT " ${SECP256K1_APPEND_CFLAGS}")
 endif()
 
+set(SECP256K1_APPEND_LDFLAGS "" CACHE STRING "Linker flags that are appended to the command line after all other flags added by the build system. This variable is intended for debugging and special builds.")
+if(SECP256K1_APPEND_LDFLAGS)
+  # Appending to this low-level rule variable is the only way to
+  # guarantee that the flags appear at the end of the command line.
+  string(APPEND CMAKE_C_CREATE_SHARED_LIBRARY " ${SECP256K1_APPEND_LDFLAGS}")
+  string(APPEND CMAKE_C_LINK_EXECUTABLE " ${SECP256K1_APPEND_LDFLAGS}")
+endif()
+
 add_subdirectory(src)
 if(SECP256K1_BUILD_EXAMPLES)
   add_subdirectory(examples)
@@ -355,6 +363,9 @@ endif()
 if(SECP256K1_APPEND_CFLAGS)
   message("SECP256K1_APPEND_CFLAGS ............... ${SECP256K1_APPEND_CFLAGS}")
 endif()
+if(SECP256K1_APPEND_LDFLAGS)
+  message("SECP256K1_APPEND_LDFLAGS .............. ${SECP256K1_APPEND_LDFLAGS}")
+endif()
 message("")
 if(print_msan_notice)
   message(