Utilities/Sphinx: Port qthelp css workaround to Sphinx 1.4

The `default.css` static content is no longer placed in the qthelp
`_static` directory.  If it does not exist, skip over it and insert
the content of `basic.css` directly into `cmake.css`.

Utilities/Sphinx/apply_qthelp_css_workaround.cmake

 
 file(READ "${CSS_DIR}/basic.css" BasicCssContent)
 
-file(READ "${CSS_DIR}/default.css" DefaultCssContent)
-string(REPLACE
-  "@import url(\"basic.css\")" "${BasicCssContent}"
-  DefaultCssContent "${DefaultCssContent}"
-)
+if(EXISTS "${CSS_DIR}/default.css")
+  file(READ "${CSS_DIR}/default.css" DefaultCssContent)
+  string(REPLACE
+    "@import url(\"basic.css\")" "${BasicCssContent}"
+    DefaultCssContent "${DefaultCssContent}"
+  )
+else()
+  set(DefaultCssContent "${BasicCssContent}")
+endif()
 
 file(READ "${CSS_DIR}/cmake.css" CMakeCssContent)
 string(REPLACE