Fix C++ amalgamation errors

[SLieve]

The C++ amalgamation example was broken, see #545 (comment). This PR fixes the two errors: explicit casting and redefining htobe64.

I've moved the big endian conversion macros into portability.h, as that seemed like a better home anyway. In the process I've renamed them to avoid naming conflicts (e.g. with <endian.h>).

Tested:

$ ./amalgamation.sh
We are about to amalgamate all CRoaring files into one source file.
For rationale, see: https://www.sqlite.org/amalgamation.html
and: https://en.wikipedia.org/wiki/Single_Compilation_Unit

Creating roaring.h...
Creating roaring.c...
Creating amalgamation_demo.c...
Creating roaring.hh...
Creating amalgamation_demo.cpp...

Files have been written to current directory: /home/soerian/repos/CRoaring
-rw-rw-r-- 1 soerian soerian    825 Jan  9 23:05 amalgamation_demo.c
-rw-rw-r-- 1 soerian soerian   2526 Jan  9 23:05 amalgamation_demo.cpp
-rw-rw-r-- 1 soerian soerian 951043 Jan  9 23:05 roaring.c
-rw-rw-r-- 1 soerian soerian  87349 Jan  9 23:05 roaring.h
-rw-rw-r-- 1 soerian soerian 107166 Jan  9 23:05 roaring.hh

The interface is found in the file 'include/roaring/roaring.h'.

For C, try:
cc -O3 -std=c11  -o amalgamation_demo amalgamation_demo.c  && ./amalgamation_demo

For C++, try:
c++ -O3 -std=c++11 -o amalgamation_demo amalgamation_demo.cpp  && ./amalgamation_demo

You can build a shared library with the following command:
cc -O3 -std=c11 -shared -o libroaring.so -fPIC roaring.c

$ cc -O3 -std=c11  -o amalgamation_demo amalgamation_demo.c  && ./amalgamation_demo
cardinality = 900
cardinality (64-bit) = 900
1000

$ c++ -O3 -std=c++11 -o amalgamation_demo amalgamation_demo.cpp  && ./amalgamation_demo

cardinality = 900
cardinality = 900

[Dr-Emann]

Nice, I do think portability.h is the right place for those macros anyway.

[lemire]

Merging.