configure: check for dlsym as well

For some reason, when compiling with gcc-asan and a recent enough gcc
version(seen on 5.3+ so far), linking dlopen works without -ldl, but
dlsym fails with:

undefined reference to symbol 'dlsym@@GLIBC_2.2.5'

So this patchs checks for both dlopen and dlsym to work for determining
if -ldl is needed.

configure

@@ -5379,9 +5379,9 @@ check_code cc arm_neon.h "int16x8_t test = vdupq_n_s16(0)" && enable intrinsics_
 check_ldflags -Wl,--as-needed
 check_ldflags -Wl,-z,noexecstack
 
-if check_func dlopen; then
+if check_func dlopen && check_func dlsym; then
     ldl=
-elif check_func dlopen -ldl; then
+elif check_func dlopen -ldl && check_func dlsym -ldl; then
     ldl=-ldl
 fi