Do not use ?= for defining macros

To define make macros one may use make arguments, so inheriting them from
environment is not needed. I also do not see nmake supporting `?=` (not sure
that anybody will use this though).

docs/Makefile

@@ -1,8 +1,8 @@
 # Makefile for Sphinx documentation
-SPHINXOPTS    ?=
-SPHINXBUILD   ?= sphinx-build
-PAPER         ?=
-BUILDDIR      ?= _build
+SPHINXOPTS    =
+SPHINXBUILD   = sphinx-build
+PAPER         =
+BUILDDIR      = _build
 
 # Internal variables
 PAPEROPT_a4     = -D latex_paper_size=a4