Made Makefile more configurable.

Moved user configurable vars to Makefile.conf.template. User can
overwrite configuration variables by setting them in Makefile.conf.
Added printconf target to show build config.
Setting defaults based on current platform (uname -s).
Checking if command can be executed using 'command -v X'. "test -x CMD"
checks only whether a filepath is executable, but doesn't work for
simple commands.

Conflicts:
	Makefile
Setting MAKEINFO only if it's not already set.
Added warning when user has no Makefile.conf.
Removed setting defaults for other platforms.

Don't know whether other platforms have similar defaults.
If so, they can be added subsequently.
Described how to customize Makefile.
Added TEXI2PDF var and shortened one description.
Described Makefile customization in ecb.texi.

The description has been added in the Installation section, just after
"Standard ECB-Installation" subsection, which has a link to "Makefile
customization".
Loading the defaults also for Cygwin and Darwin.
Merge pull request #16 from pwojnowski/master

Moved user options form Makefile to Makefile.conf