psl: require libpsl 0.16.0 (2016-12-10) or greater

Debian Stretch offers 0.17.0.

Ref: https://github.com/rockdaboot/libpsl/releases/tag/libpsl-0.16.0
Ref: https://sources.debian.org/src/libpsl/

Closes #21933

docs/INTERNALS.md

@@ -31,6 +31,7 @@ We aim to support these or later versions.
 - GnuTLS       3.6.5 (2018-12-01)
 - libidn2      2.0.0 (2017-03-29)
 - libgsasl     1.6.0 (2010-12-14)
+- libpsl       0.16.0 (2016-12-10)
 - LibreSSL     2.9.1 (2019-04-22)
 - libssh       0.9.0 (2019-06-28)
 - libssh2      1.9.0 (2019-06-20)

lib/psl.c

@@ -29,6 +29,10 @@
 #include "progress.h"
 #include "curl_share.h"
 
+#if !defined(PSL_VERSION_NUMBER) || PSL_VERSION_NUMBER < 0x001000
+#error "libpsl 0.16.0 or greater required"
+#endif
+
 void Curl_psl_destroy(struct PslCache *pslcache)
 {
   if(pslcache->psl) {
@@ -65,7 +69,6 @@ const psl_ctx_t *Curl_psl_use(struct Curl_easy *easy)
       bool dynamic = FALSE;
       time_t expires = TIME_T_MAX;
 
-#if defined(PSL_VERSION_NUMBER) && PSL_VERSION_NUMBER >= 0x001000
       psl = psl_latest(NULL);
       dynamic = psl != NULL;
       /* Take care of possible time computation overflow. */
@@ -74,8 +77,6 @@ const psl_ctx_t *Curl_psl_use(struct Curl_easy *easy)
 
       /* Only get the built-in PSL if we do not already have the "latest". */
       if(!psl && !pslcache->dynamic)
-#endif
-
         psl = psl_builtin();
 
       if(psl) {