cmake: default to +ownlibs to improve build times (#11483)

- CMake is built *very* frequently, and the number of dependencies it has
  accumulated can take a very long time on some systems.

- Use +ownlibs by default to avoid the build overhead. This won't
  interfere with other packages by introducing conflicting libraries, as
  CMake is nearly always a build dependency.

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

@@ -16,7 +16,7 @@ class Cmake(Package):
     version('3.14.2', sha256='a3cbf562b99270c0ff192f692139e98c605f292bfdbc04d70da0309a5358e71e')
     version('3.14.1', sha256='7321be640406338fc12590609c42b0fae7ea12980855c1be363d25dcd76bb25f')
     version('3.14.0', sha256='aa76ba67b3c2af1946701f847073f4652af5cbd9f141f221c97af99127e75502')
-    version('3.13.4',   'fdd928fee35f472920071d1c7f1a6a2b72c9b25e04f7a37b409349aef3f20e9b') 
+    version('3.13.4',   'fdd928fee35f472920071d1c7f1a6a2b72c9b25e04f7a37b409349aef3f20e9b')
     version('3.13.3',   '665f905036b1f731a2a16f83fb298b1fb9d0f98c382625d023097151ad016b25')
     version('3.13.2',   'c925e7d2c5ba511a69f43543ed7b4182a7d446c274c7480d0e42cd933076ae25')
     version('3.13.1',   'befe1ce6d672f2881350e94d4e3cc809697dd2c09e5b708b76c1dae74e1b2210')
@@ -59,7 +59,11 @@ class Cmake(Package):
     # See https://gitlab.kitware.com/cmake/cmake/merge_requests/2873
     patch('cmake-macos-add-coreservices.patch', when='@3.11.0:3.13.3')
 
-    variant('ownlibs', default=False,  description='Use CMake-provided third-party libraries')
+    # We default ownlibs to true because it greatly speeds up the CMake
+    # build, and CMake is built frequently. Also, CMake is almost always
+    # a build dependency, and its libs will not interfere with others in
+    # the build.
+    variant('ownlibs', default=True,  description='Use CMake-provided third-party libraries')
     variant('qt',      default=False, description='Enables the build of cmake-gui')
     variant('doc',     default=False, description='Enables the generation of html and man page documentation')
     variant('openssl', default=True,  description="Enables CMake's OpenSSL features")