nginx: add 1.15.6 (#9780)

Needed to compile with gcc@8 and recent versions of glibc.

var/spack/repos/builtin/packages/nginx/package.py

@@ -14,6 +14,7 @@ class Nginx(AutotoolsPackage):
     homepage = "https://nginx.org/en/"
     url      = "https://nginx.org/download/nginx-1.12.0.tar.gz"
 
+    version('1.15.6', 'a3d8c67c2035808c7c0d475fffe263db8c353b11521aa7ade468b780ed826cc6')
     version('1.13.8', 'df4be9294365782dc1349ca33ce8c4ac')
     version('1.12.0', '995eb0a140455cf0cfc497e5bd7f94b3')
 
@@ -21,6 +22,8 @@ class Nginx(AutotoolsPackage):
     depends_on('pcre')
     depends_on('zlib')
 
+    conflicts('%gcc@8:', when='@:1.14')
+
     def configure_args(self):
         args = ['--with-http_ssl_module']
         return args