Update package: add info for emacs@26.1 (#8302)

* Update package: add info for emacs@26.1

In addition to the digest, it needs gnutls (perhaps there's a way to
turn it off, but it seems like a Good Thing to encourage, so...).

* Clean up tls variant handling

- Don't to specify the dependency for @26.1:, just need to fix what's
  already there.
- Don't assume that configure will DTRT w.r.t. gnutls when it's
  discovered in the environmnet, use explicit configure switches.
- Clean up some unnecessary dependencies when +tls.  Why would you
  need libxpm when building +tls?  Will ask the original author about
  this.

* Use append when adding configure switches

* Add dependency on pcre

When one builds +X, Spack's pcre is pulled into the build via
the X dependencies.  When one builds without it, the build discovers
and links against the system library.

We might as well just depend on it and have a consistent outcome.

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

@@ -33,6 +33,7 @@ class Emacs(AutotoolsPackage):
     homepage = "https://www.gnu.org/software/emacs"
     url      = "http://ftp.gnu.org/gnu/emacs/emacs-24.5.tar.gz"
 
+    version('26.1', '544d2ab5eb142e9ca69adb023d17bf4b')
     version('25.3', '74ddd373dc52ac05ca7a8c63b1ddbf58')
     version('25.2', '0a36d1cdbba6024d4dbbac027f87995f')
     version('25.1', '95c12e6a9afdf0dcbdd7d2efa26ca42c')
@@ -50,6 +51,7 @@ class Emacs(AutotoolsPackage):
     depends_on('pkgconfig', type='build')
 
     depends_on('ncurses')
+    depends_on('pcre')
     depends_on('zlib')
     depends_on('libtiff', when='+X')
     depends_on('libpng', when='+X')
@@ -59,8 +61,6 @@ class Emacs(AutotoolsPackage):
     depends_on('libxaw', when='+X toolkit=athena')
     depends_on('gtkplus', when='+X toolkit=gtk')
     depends_on('gnutls', when='+tls')
-    depends_on('libxpm ^gettext+libunistring', when='+tls')
-    depends_on('ncurses+termlib', when='+tls')
 
     def configure_args(self):
         spec = self.spec
@@ -79,4 +79,9 @@ def configure_args(self):
         if sys.platform == 'darwin':
             args.append('--without-ns')
 
+        if '+tls' in spec:
+            args.append('--with-gnutls')
+        else:
+            args.append('--without-gnutls')
+
         return args