http://sourceforge.net/p/fteqw/code/HEA ... er.c#l2107
side note: using the word shader for both materials and glsl code is stupid. oh well.
the s_normalmap stuff comes from a reworking I made 1-2 months ago. the engine automatically inserts the map lines/blocks into your shader based upon the specially-named textures in your glsl. this means you don't need map $diffuse, map $normalmap, map $deluxemap, map $lightmap, map $fullbright, map $upper, map $lower... you get the idea. this separates the shader from the glsl a little more which can only be a good thing (the worst part was that the order needed to match).
internally the engine has a shader override system, this includes stuff like the rtlight shader. these shaders 'inherit' their named s_diffuse/$diffuse etc textures from the original shader that they are replacing, which is how fte uses one rtlight shader (with permutations based upon texture availability) for pretty much everything.
naturally you can use 'bemode rtlight othershader' to specify a different override for the rtlight passes. there's a few other backend modes that you can provide alternative shaders for like that. the terrain system does this too, of course (which uses the lightmap to mix 4 underlying textures, which means it can't use the regular rtlight logic as that assumes there is only a single diffusemap).
unfortunately not all surfaces define all texture maps (or might define them but fail to load them perhaps). unspecified texture maps often have some awkward default of 0,0,0,1 (which is why s_lightmap is meant to end up as 1,1,1,1).
note that certain bsp formats(rbsp+fbsp, but not q3bsp) have multiple lightmaps per face (now introducing the LIGHTSTYLED permutation!)
You should probably figure out the various permutations in order to avoid this from being an issue (as well as to avoid wasting gpu time on logic that just isn't relevant).
e_lmscale can end up as 0 if r_shadow_realtime_world_lightmap is 0.
v_lmcoord is a vertex attribute and is thus only valid in the vertex shader. e_lmscale is a uniform and should be usable in either (but usefully the fragment shader). both should be included in the "sys/defs.h" header