Fixed building python module with clang 8.0

C++20 has an include file named 'version' that is included in
cstddef with `include <version>'.

On OSX, case insensitive, file python/VERSION is included by mistake.
The problem appears when compiling with clang 8.

I noticed this when building conda package on travis, but it can be
reproduced e.g. using

cd python && make clean && make pip && CC=clang-mp-8.0 python3.7 setup.py build_ext -i

I fixed this putting Plumed.h in a separate directory

python/MANIFEST.in

 include VERSION
-include Plumed.h
+include include/Plumed.h
 include plumed.pyx
 include cplumed.pxd

python/Makefile

@@ -28,8 +28,9 @@ all:
 endif
 
 pip:
-	cp ../src/wrapper/Plumed.h .
 	cp ../VERSION .
+	mkdir -p include
+	cp ../src/wrapper/Plumed.h include/
 
 ifndef python_bin
 python_bin=python
@@ -39,10 +40,12 @@ endif
 
 pypi:
 	mkdir -p pypi
+	mkdir -p pypi/include
 	cp -r test pypi/
-	cp ../src/wrapper/Plumed.h README.rst MANIFEST.in cplumed.pxd plumed.pyx setup.py ../VERSION pypi/
+	cp ../src/wrapper/Plumed.h pypi/include/
+	cp README.rst MANIFEST.in cplumed.pxd plumed.pyx setup.py ../VERSION pypi/
 	cd pypi ; $(python_bin) setup.py sdist
 	echo "now use: $(python_bin) -m twine upload dist/plumed-$(VERSION).tar.gz"
 
 clean:
-	rm -fr *.so plumed.cpp build Plumed.h VERSION pypi test/logfile test/bck* pypi
+	rm -fr *.so plumed.cpp build Plumed.h VERSION pypi test/logfile test/bck* pypi include

python/setup.py

@@ -39,7 +39,11 @@ def is_platform_mac():
 
 if os.getenv("plumed_macports") is not None:
     copyfile("../VERSION","VERSION")
-    copyfile("../src/wrapper/Plumed.h","Plumed.h")
+    try:
+        os.mkdir("include")
+    except OSError:
+        pass
+    copyfile("../src/wrapper/Plumed.h","include/Plumed.h")
 
 plumedname = os.getenv("plumed_program_name")
 if plumedname is None:
@@ -80,7 +84,7 @@ def readme():
 try:
     include_dirs=[os.environ["plumed_include_dir"]]
 except KeyError:
-    include_dirs=["."]
+    include_dirs=["./include"]
 
 # allow one to force using cython with env var plumed_force_cython=yes
 USE_CYTHON = False