Improve Makefile compatibility and build docs

We now use `$(MAKE)` for subsequent calls to `make`.
This means that systems that do not default to GNU make can run `gmake man` and
produce the man pages.

While there also document a dependency on the rtd theme for sphinx.

Makefile

@@ -35,9 +35,9 @@ DOCS_SRC_DIR=./docs/src
 SPEECHDIR=./docs/speeches
 TYPEDIR=./cdist/conf/type
 
-SPHINXM=make -C $(DOCS_SRC_DIR) man
-SPHINXH=make -C $(DOCS_SRC_DIR) html
-SPHINXC=make -C $(DOCS_SRC_DIR) clean
+SPHINXM=$(MAKE) -C $(DOCS_SRC_DIR) man
+SPHINXH=$(MAKE) -C $(DOCS_SRC_DIR) html
+SPHINXC=$(MAKE) -C $(DOCS_SRC_DIR) clean
 
 ################################################################################
 # Manpages

docs/src/cdist-install.rst

@@ -12,7 +12,7 @@ This is the machine from which you will configure target hosts.
  * /bin/sh: A POSIX like shell (for instance bash, dash, zsh)
  * Python >= 3.5
  * SSH client
- * sphinx (for building html docs and/or the man pages)
+ * sphinx with the rtd theme (for building html docs and/or the man pages)
 
 Target Hosts
 ~~~~~~~~~~~~