Add missing include files in .c files, add missing prototype in .h and activate the warning for implicit functions.
Remove all the relative path in the .h #include and add the folder paths in makefile.
Merge pull request #67 from ihbar/FixInclude

Add missing include files in .c files, add missing prototype in .h
Fix comilation on non-Pi systems

Move the -I include path and -Wimplicit flag to a common CFLAGS
option to fix compilation on non-Pi systems as well as enable
implicit include warnings when compiling on those systems.
Make some args const

Make the args passed to FileExists() and DirectoryExists() const.