build: unify mesa version by using a VERSION file

Rather than having to keep track of all the build systems and their respecitve
definition of the mesa version, use a single top file VERSION. Every build
system is responsible for reading/parsing the file and using it

v2:
* remove useless bulletpoint from the documentation, suggested by Matt
* "Androing is Linux. Use '/' in stead of '\'", spotted by Chad V
* use cleaner code to get the version in scons, suggested by Chad V

v3:
* ensure leading and trailing whitespace characters are stripped while parsing
* android: handle GNU shell commands approapriately

Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Chad Versace <chad.versace@linux.intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>

VERSION

@@ -0,0 +1 @@
+9.3.0-devel