Fix detection of "otim".

Wrong use of parentheses caused condition to be meaningless (my bad).
Do not install PORTING file.
Use AM_MAINTAINER_MODE.
Book: mark more functions static.
Use SEEK_SET instead of literal 0 in lseek calls.
Make ShowMessage stdarg-enabled.
Warnings: be explicit about values we are using.
Warnings: correctly use #ifdef for declarations.
Warnings: format string issue.
Issue a note when "bsave" is aborted because of missing filename.
Book: replace *Offset macros with functions.
Book: replace Write* macros with functions.
Book: simplify WriteData.
Book: get rid of currentoffset as a global var, be explicit instead.
Book: always use Write* API to write into binbook.

Makes it easier to factorise error handling.
InputCommand: format alternatives consistently.