Fix QT4 build for mac (#10944)

This updates the Spack QT package to enable building qt version 4 on
MacOS. 

This includes the following changes to the qt package:

* add version 4.8.7
* add option to build with or without shared libs
* add options to disable tools, ssl, sql, and freetype support
* add qt4-tools patch when building qt@4+tools
* add option to build as a framework (only available on MacOS)
* replace qt4-el-capitan patch with qt4-mac patch (which includes the
  edits from qt4-el-capitan)
* apply qt4-pcre-include-conflict.patch only for version 4.8.6
  (rather than all 4.x versions)
* apply qt4-gcc-and-webkit.patch for 4.x versions before 4.8.7 and
  create a separate qt4-gcc-and-webkit-487.patch for version 4.8.7
* update patch function for qt@4 on MacOS to update configure
  variables relevant to Spack (e.g. PREFIX)
* add option to build freetype with Spack, as a vendored dependency
  of QT, or not at all (default is to build with Spack)

This includes the following edits outside of the qt package:

* Update MacOS version utility function to return all parts of the
  Mac version (rather than just the first two)
* gettext package: implement "libs"
* python package: add gettext as a dependency

lib/spack/spack/operating_systems/mac_os.py

@@ -14,7 +14,7 @@
 def macos_version():
     """temporary workaround to return a macOS version as a Version object
     """
-    return Version('.'.join(py_platform.mac_ver()[0].split('.')[:2]))
+    return Version(py_platform.mac_ver()[0])
 
 
 def macos_sdk_path():
@@ -46,7 +46,7 @@ def __init__(self):
                         "10.13": "highsierra",
                         "10.14": "mojave"}
 
-        mac_ver = '.'.join(py_platform.mac_ver()[0].split('.')[:2])
+        mac_ver = str(macos_version().up_to(2))
         name = mac_releases.get(mac_ver, "macos")
         super(MacOs, self).__init__(name, mac_ver)