Compute each Makefile variable immediately

Otherwise the variables get re-computed over and over again every time
they are used, slowing things up.  Especially if someone adapts the
Makefile to several targets in their own real project. :)

Demos/embed/Makefile

 # Makefile for creating our standalone Cython program
-PYTHON=python
-PYVERSION=$(shell $(PYTHON) -c "import sys; print(sys.version[:3])")
+PYTHON := python
+PYVERSION := $(shell $(PYTHON) -c "import sys; print(sys.version[:3])")
 
-INCDIR=$(shell $(PYTHON) -c "from distutils import sysconfig; print(sysconfig.get_python_inc())")
-PLATINCDIR=$(shell $(PYTHON) -c "from distutils import sysconfig; print(sysconfig.get_python_inc(plat_specific=True))")
-LIBDIR1=$(shell $(PYTHON) -c "from distutils import sysconfig; print(sysconfig.get_config_var('LIBDIR'))")
-LIBDIR2=$(shell $(PYTHON) -c "from distutils import sysconfig; print(sysconfig.get_config_var('LIBPL'))")
-PYLIB=$(shell $(PYTHON) -c "from distutils import sysconfig; print(sysconfig.get_config_var('LIBRARY')[3:-2])")
+INCDIR := $(shell $(PYTHON) -c "from distutils import sysconfig; print(sysconfig.get_python_inc())")
+PLATINCDIR := $(shell $(PYTHON) -c "from distutils import sysconfig; print(sysconfig.get_python_inc(plat_specific=True))")
+LIBDIR1 := $(shell $(PYTHON) -c "from distutils import sysconfig; print(sysconfig.get_config_var('LIBDIR'))")
+LIBDIR2 := $(shell $(PYTHON) -c "from distutils import sysconfig; print(sysconfig.get_config_var('LIBPL'))")
+PYLIB := $(shell $(PYTHON) -c "from distutils import sysconfig; print(sysconfig.get_config_var('LIBRARY')[3:-2])")
 
-CC=$(shell $(PYTHON) -c "import distutils.sysconfig; print(distutils.sysconfig.get_config_var('CC'))")
-LINKCC=$(shell $(PYTHON) -c "import distutils.sysconfig; print(distutils.sysconfig.get_config_var('LINKCC'))")
-LINKFORSHARED=$(shell $(PYTHON) -c "import distutils.sysconfig; print(distutils.sysconfig.get_config_var('LINKFORSHARED'))")
-LIBS=$(shell $(PYTHON) -c "import distutils.sysconfig; print(distutils.sysconfig.get_config_var('LIBS'))")
-SYSLIBS= $(shell $(PYTHON) -c "import distutils.sysconfig; print(distutils.sysconfig.get_config_var('SYSLIBS'))")
+CC := $(shell $(PYTHON) -c "import distutils.sysconfig; print(distutils.sysconfig.get_config_var('CC'))")
+LINKCC := $(shell $(PYTHON) -c "import distutils.sysconfig; print(distutils.sysconfig.get_config_var('LINKCC'))")
+LINKFORSHARED := $(shell $(PYTHON) -c "import distutils.sysconfig; print(distutils.sysconfig.get_config_var('LINKFORSHARED'))")
+LIBS := $(shell $(PYTHON) -c "import distutils.sysconfig; print(distutils.sysconfig.get_config_var('LIBS'))")
+SYSLIBS :=  $(shell $(PYTHON) -c "import distutils.sysconfig; print(distutils.sysconfig.get_config_var('SYSLIBS'))")
 
 embedded: embedded.o
 	$(LINKCC) -o $@ $^ -L$(LIBDIR1) -L$(LIBDIR2) -l$(PYLIB) $(LIBS) $(SYSLIBS) $(LINKFORSHARED)
@@ -20,7 +20,7 @@ embedded: embedded.o
 embedded.o: embedded.c
 	$(CC) -c $^ -I$(INCDIR) -I$(PLATINCDIR)
 
-CYTHON=../../cython.py
+CYTHON := ../../cython.py
 embedded.c: embedded.pyx
 	@$(PYTHON) $(CYTHON) --embed embedded.pyx