Don't rely on the OpenSSL version to detect ALPN

A vendor may back-port the feature to an older release, so we might as
well look for the presence of one of its basic functions. This includes
NPN since the code uses them in tandem, and keeps the ability to exclude
NPN with the OPENSSL_NO_NEXTPROTONEG macro since OpenSSL itself relies
on that internally.

Except that currently Hitch won't build when OPENSSL_NO_NEXTPROTONEG is
defined, which was already the case before this change.

configure.ac

@@ -162,6 +162,14 @@ fi
 AC_CHECK_HEADERS([linux/futex.h])
 AM_CONDITIONAL([HAVE_LINUX_FUTEX], [test $ac_cv_header_linux_futex_h = yes])
 
+HITCH_CHECK_FUNC([SSL_get0_alpn_selected], [$SSL_LIBS], [
+	AC_DEFINE([OPENSSL_WITH_ALPN], [1], [OpenSSL supports ALPN])
+])
+
+HITCH_CHECK_FUNC([SSL_get0_next_proto_negotiated], [$SSL_LIBS], [
+	AC_DEFINE([OPENSSL_WITH_NPN], [1], [OpenSSL supports NPN])
+])
+
 SH_TESTS="$(cd $srcdir/src && echo tests/test*.sh)"
 AC_SUBST(SH_TESTS)
 

hitch.m4

@@ -1,25 +1,34 @@
 # HITCH_SEARCH_LIBS(VAR, LIBS, FUNC, NOTFOUND)
 # --------------------------------------------
 AC_DEFUN([HITCH_SEARCH_LIBS], [
-	hitch_save_LIBS="${LIBS}"
+	hitch_save_LIBS=$LIBS
 	LIBS=""
 	AC_SEARCH_LIBS([$3], [$2], [], [$4])
 	AC_SUBST([$1_LIBS], [$LIBS])
-	LIBS="${hitch_save_LIBS}"
+	LIBS=$hitch_save_LIBS
+])
+
+# HITCH_CHECK_FUNC(FUNC, LIBS, FOUND, NOTFOUND)
+# --------------------------------------------
+AC_DEFUN([HITCH_CHECK_FUNC], [
+	hitch_save_LIBS=$LIBS
+	LIBS="$2"
+	AC_CHECK_FUNC([$1], [$3], [$4])
+	LIBS=$hitch_save_LIBS
 ])
 
 # _HITCH_CHECK_FLAG(VAR, FLAG)
 ------------------------------
 AC_DEFUN([_HITCH_CHECK_FLAG], [
 
 	AC_MSG_CHECKING([whether the compiler accepts $2])
-	_cflags="$CFLAGS"
+	hitch_save_CFLAGS=$CFLAGS
 	CFLAGS="[$]$1 $2 $CFLAGS"
 	AC_RUN_IFELSE(
 		[AC_LANG_SOURCE([int main(void) { return (0); }])],
 		[AC_MSG_RESULT([yes]); $1="[$]$1 $2"],
 		[AC_MSG_RESULT([no])])
-	CFLAGS="$_cflags"
+	CFLAGS=$hitch_save_CFLAGS
 ])
 
 # HITCH_CHECK_FLAGS(VAR, FLAGS)

src/configuration.h

@@ -13,16 +13,9 @@
 
 #include "foreign/uthash.h"
 
-/* Is NPN available? See openssl/opensslv.h for explanation. */
-#ifndef OPENSSL_NO_NEXTPROTONEG
-#  if OPENSSL_VERSION_NUMBER >= 0x1000100fL
-#    define OPENSSL_WITH_NPN
-#  endif
-#endif
-
-/* Is ALPN available? See openssl/opensslv.h for explanation. */
-#if OPENSSL_VERSION_NUMBER >= 0x1000200fL
-#  define OPENSSL_WITH_ALPN
+/* This macro disables NPN even in openssl/ssl.h */
+#ifdef OPENSSL_NO_NEXTPROTONEG
+#  undef OPENSSL_WITH_NPN
 #endif
 
 #ifdef OPENSSL_WITH_ALPN