dillo: fix build on macOS <10.10

* blacklist clang < 700

Co-authored-by: Sergey Fedorov <barracuda@macos-powerpc.org>

www/dillo/Portfile

@@ -4,6 +4,9 @@ PortSystem              1.0
 PortGroup               legacysupport 1.1
 PortGroup               openssl 1.0
 
+# clock_gettime
+legacysupport.newest_darwin_requires_legacy 15
+
 name                    dillo
 version                 3.3.0
 revision                0
@@ -36,5 +39,11 @@ post-patch {
 # Experimental support for FLTK v1.4+ is currently required.
 configure.args-append   --enable-experimental-fltk
 
+# error: call to implicitly-deleted default constructor of
+# 'const CssPropertyInfo' on macOS <10.10
+compiler.c_standard     1999
+compiler.blacklist-append \
+                        {clang < 700}
+
 livecheck.url           ${homepage}/release/
 livecheck.regex         {>Version ([0-9.]+)<}