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

@@ -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")