neovim.rb: modernize the style

https://github.com/styleguide/ruby
neovim.rb: improve dependency

1. remove unnecessary deps `md5sha1sum`
2. add `=> :build`
3. add missing deps `autoconf`
Merge pull request #922 'Improve neovim.rb'
Replace int with bool in some files. #654
Replaced most TRUE/FALSE macros in arabic, mbyte and spell. #645
remove proto.h include from vim.h
remove stdbool.h include from vim.h and globals.h
move memory.h include to version.c
move ascii.h include out of vim.h
move locale.h include out of vim.h
move <inttypes.h> include out of vim.h
move wctype.h include out of vim.h
move assert.h include out of vim.h
move stdarg.h include out of vim.h
move errno.h include out of vim.h
Merge #921 'Remove includes vim.h'
Fix two android compile errors in fs.c #924

Replace usage of deprecated S_IEXEC with S_IXUSR.
Rename a variable named "errno" to avoid clashing with define.
Avoid linking with libform(w).so

If libtgent or libcurses is installed, the first one found of them
is linked to.

But if not, a find_package(Curses REQUIRED) is used and CURSES_LIBRARIES
is added to NVIM_LINK_LIBRARIES. This contains libform(w).so on many
systems, causing nvim to be linked to and depend on libform(w).so,
which may not be installed one some space-constrained systems, unnecessarily.
Merge #933 'Avoid linking with libform(w).so'
Default 'encoding'-option from latin to utf-8 #935

Normally the default encoding does not have much effect, since it's
overridden by the environment.

But when it's not (test with "LANG= LC_ALL= C_CTYPE= nvim" and perform
":set encoding?"), utf-8 should be the default encoding for a 21st
century editor :).