Attempt minimal autoconf changes to use GnuTLS instead of OpenSSL.

I could waste the rest of the available time trying to turn configure.ac into something that cleanly ignores OpenSSL, uses GnuTLS instead and retains all the options. Or even better scrap the whole autoconf mess and start again.

But in the interests of prototyping, do something quick and dirty. This means GnuTLS must for now be configured thus:

$ CFLAGS="-g" ../configure --enable-stub-only --with-gnutls --disable-gost --disable-ecdsa --disable-edns-cookies

to evade other items with hardcoded OpenSSL checks in them.

configure.ac

@@ -399,11 +399,26 @@ yes)
 	;;
 esac
 
+# Which TLS and crypto libs to use.
+AC_ARG_WITH([gnutls],
+    [AS_HELP_STRING([--with-gnutls],
+        [use GnuTLS instead of OpenSSL])],
+    [
+        PKG_CHECK_MODULES([libgnutls], [gnutls >= 3.5.0])
+        LIBS="$libgnutls_LIBS $LIBS"
+        CFLAGS="$libgnutls_CFLAGS $CFLAGS"
+        AC_SUBST([TLSDIR], 'gnutls')
+        AC_DEFINE([USE_GNUTLS], [1], [Use the GnuTLS library])
+    ],
+    [
+        ACX_WITH_SSL_OPTIONAL
+        ACX_LIB_SSL
+        AC_SUBST([TLSDIR], 'openssl')
+    ])
+
 USE_NSS="no"
 # openssl
 if test $USE_NSS = "no"; then
-ACX_WITH_SSL_OPTIONAL
-ACX_LIB_SSL
 AC_MSG_CHECKING([for LibreSSL])
 if grep VERSION_TEXT $ssldir/include/openssl/opensslv.h | grep "LibreSSL" >/dev/null; then
 	AC_MSG_RESULT([yes])

src/Makefile.in

@@ -52,11 +52,12 @@ INSTALL = @INSTALL@
 INSTALL_DATA = @INSTALL_DATA@
 
 srcdir = @srcdir@
+tlsdir = @TLSDIR@
 stubbysrcdir = $(srcdir)/../stubby
 LIBTOOL = ../libtool
 
 CC=@CC@
-CFLAGS=-I$(srcdir) -I. -I$(srcdir)/util/auxiliary -I$(srcdir)/openssl -I$(stubbysrcdir)/src @CFLAGS@ @CPPFLAGS@ $(XTRA_CFLAGS)
+CFLAGS=-I$(srcdir) -I. -I$(srcdir)/util/auxiliary -I$(srcdir)/$(tlsdir) -I$(stubbysrcdir)/src @CFLAGS@ @CPPFLAGS@ $(XTRA_CFLAGS)
 WPEDANTICFLAG=@WPEDANTICFLAG@
 WNOERRORFLAG=@WNOERRORFLAG@
 LDFLAGS=@LDFLAGS@ @LIBS@
@@ -134,7 +135,7 @@ $(JSMN_OBJ):
 	$(LIBTOOL) --quiet --tag=CC --mode=compile $(CC) $(CFLAGS) -DJSMN_GETDNS -c $(srcdir)/jsmn/$(@:.lo=.c) -o $@
 
 $(TLS_OBJ):
-	$(LIBTOOL) --quiet --tag=CC --mode=compile $(CC) $(CFLAGS) -c $(srcdir)/openssl/$(@:.lo=.c) -o $@
+	$(LIBTOOL) --quiet --tag=CC --mode=compile $(CC) $(CFLAGS) -c $(srcdir)/$(tlsdir)/$(@:.lo=.c) -o $@
 
 $(YAML_OBJ):
 	$(LIBTOOL) --quiet --tag=CC --mode=compile $(CC) $(CFLAGS) -c $(stubbysrcdir)/src/yaml/$(@:.lo=.c) -o $@
@@ -146,7 +147,7 @@ $(EXTENSION_OBJ):
 	$(LIBTOOL) --quiet --tag=CC --mode=compile $(CC) $(CFLAGS) $(WPEDANTICFLAG) -c $(srcdir)/extension/$(@:.lo=.c) -o $@
 
 anchor.lo:
-	$(LIBTOOL) --quiet --tag=CC --mode=compile $(CC) $(CFLAGS) $(WPEDANTICFLAG) $(C99COMPATFLAGS) -c $(srcdir)/openssl/anchor.c -o anchor.lo
+	$(LIBTOOL) --quiet --tag=CC --mode=compile $(CC) $(CFLAGS) $(WPEDANTICFLAG) $(C99COMPATFLAGS) -c $(srcdir)/$(tlsdir)/anchor.c -o anchor.lo
 
 context.lo:
 	$(LIBTOOL) --quiet --tag=CC --mode=compile $(CC) $(CFLAGS) $(WPEDANTICFLAG) $(C99COMPATFLAGS) -c $(srcdir)/context.c -o context.lo
@@ -271,14 +272,14 @@ Makefile: $(srcdir)/Makefile.in ../config.status
 depend:
 	(cd $(srcdir) ; awk 'BEGIN{P=1}{if(P)print}/^# Dependencies/{P=0}' Makefile.in > Makefile.in.new )
 
-	(blddir=`pwd`; cd $(srcdir) ; gcc -MM -I. -I"$$blddir" -Iopenssl -Iyxml -Iutil/auxiliary -I../stubby/src *.c gldns/*.c compat/*.c util/*.c jsmn/*.c openssl/*.c yxml/*.c extension/*.c ../stubby/src/*.c | \
+	(blddir=`pwd`; cd $(srcdir) ; gcc -MM -I. -I"$$blddir" -I$(tlsdir) -Iyxml -Iutil/auxiliary -I../stubby/src *.c gldns/*.c compat/*.c util/*.c jsmn/*.c $(tlsdir)/*.c yxml/*.c extension/*.c ../stubby/src/*.c | \
 		sed -e "s? $$blddir/? ?g" \
 		    -e 's? gldns/? $$(srcdir)/gldns/?g' \
 		    -e 's? compat/? $$(srcdir)/compat/?g' \
 		    -e 's? util/auxiliary/util/? $$(srcdir)/util/auxiliary/util/?g' \
 		    -e 's? util/? $$(srcdir)/util/?g' \
 		    -e 's? jsmn/? $$(srcdir)/jsmn/?g' \
-		    -e 's? openssl/? $$(srcdir)/openssl/?g' \
+		    -e 's? $$(tlsdir)/? $$(srcdir)/$$(tlsdir)/?g' \
 		    -e 's? yxml/? $$(srcdir)/yxml/?g' \
 		    -e 's? extension/? $$(srcdir)/extension/?g' \
 		    -e 's? \.\./stubby/? $$(stubbysrcdir)/?g' \