configure.ac: add crypto libs late

Otherwise crypto libs are linked for feature tests, which may
cause unwanted detections. For example LibreSSL publishes the
function `explicit_bzero()`, which masks the system alternative,
e.g. memset_s() on macOS. Then when trying to compile libssh2,
the declaration is missing:

```
bcrypt_pbkdf.c:93:5: error: implicit declaration of function 'explicit_bzero' is invalid in C99 [-Werror,-Wimplicit-function-declaration]
    _libssh2_explicit_zero(ciphertext, sizeof(ciphertext));
    ^
../src/misc.h:50:43: note: expanded from macro '_libssh2_explicit_zero'
                                          ^
```

Fix this by adding crypto libs to `LIBS` at the very end of the
configuration process.

Regression from 4f0f4bf

configure.ac

@@ -117,18 +117,6 @@ else
   test "$found_crypto_str" = "" && found_crypto_str="$found_crypto"
 fi
 
-if   test "$found_crypto" = "openssl"; then
-  LIBS="${LIBS} ${LTLIBSSL}"
-elif test "$found_crypto" = "wolfssl"; then
-  LIBS="${LIBS} ${LTLIBWOLFSSL}"
-elif test "$found_crypto" = "libgcrypt"; then
-  LIBS="${LIBS} ${LTLIBGCRYPT}"
-elif test "$found_crypto" = "wincng"; then
-  LIBS="${LIBS} ${LTLIBBCRYPT}"
-elif test "$found_crypto" = "mbedtls"; then
-  LIBS="${LIBS} ${LTLIBMBEDCRYPTO}"
-fi
-
 # libz
 
 AC_ARG_WITH([libz],
@@ -377,6 +365,19 @@ AM_CONDITIONAL([HOST_WINDOWS], [test "x$host_windows" = "xyes"])
 # Configure parameters
 LIBSSH2_CHECK_OPTION_WERROR
 
+# Append crypto lib
+if   test "$found_crypto" = "openssl"; then
+  LIBS="${LIBS} ${LTLIBSSL}"
+elif test "$found_crypto" = "wolfssl"; then
+  LIBS="${LIBS} ${LTLIBWOLFSSL}"
+elif test "$found_crypto" = "libgcrypt"; then
+  LIBS="${LIBS} ${LTLIBGCRYPT}"
+elif test "$found_crypto" = "wincng"; then
+  LIBS="${LIBS} ${LTLIBBCRYPT}"
+elif test "$found_crypto" = "mbedtls"; then
+  LIBS="${LIBS} ${LTLIBMBEDCRYPTO}"
+fi
+
 AC_CONFIG_FILES([Makefile
                  src/Makefile
                  tests/Makefile