configure.ac: null,android,gdi are not valid egl-platforms

... and update the documentation to reflect reality.
null and gdi are gone, and surfaceless is a recent addition.

v2: s/platforms/platform/ (spotted by Thomas)

Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Thomas Helland <thomashelland90@gmail.com>
2 files changed