cmake: Introduce SECP256K1_APPEND_LDFLAGS variable

Downstream projects may include libsecp test executables in their test
suites. In such cases, if a `-fsanitize=...` flag is passed via the
`SECP256K1_APPEND_CFLAGS` variable, the same flag must also be passed to
the linker.

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(