cmake: Support overriding Sphinx HTML doc install directory

Provide ${PROJECT}_INSTALL_SPHINX_HTML_DIR variables (e.g.
LLVM_INSTALL_SPHINX_HTML_DIR) to override Sphinx HTML doc install
directory.

Bug: https://llvm.org/bugs/show_bug.cgi?id=23780

Differential Revision: https://reviews.llvm.org/D23757

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@282240 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed