Moved chipmunk_ff.h to src/chipmunk_fii.c

Under the assumption that it does not declares a public interface of any
kind, but rather defines pointers to functions when compiling.
Fixed C-files pointing to wrong include locations
Unified FFI by using non-static inlines for public functions

A separate translation unit is now responsible for generating exports,
as recommended by section 6.7.4 of the ISO C standard

This completes the task in *include/chipmunk/chipmunk_ffi.h*:

    // TODO: get rid of the reliance on static inlines.
    // They make a mess for FFIs.

Legacy function pointers are still supported, though the use of
-DCHIPMUNK_LEGACY_FFI. An additional option in cmake makes it easy to
enable or disable this feature. Eventually, it can be removed completely.
build: -lm on UNIX systems

Just like Android, UNIX systems need to link with -lm.
put the major version in the SONAME

This makes it easier to manage the version of chipmunk. When you bump
theh major version it will automatically bump the SONAME. This is needed
for proper compatibility management.
Merge branch 'soname'

Conflicts:
	src/CMakeLists.txt
Merge branch 'unix_math'