util: automake: rework the format_srgb.c rule
util: automake: rework the format_srgb.c rule

A handful of changes/cleanups paving the way to bmake support:
 - Remove optional $(srcdir)/ prefix for files in the prereq list.
 - Drop the space after the AM_V_GEN variable.
 - Using $< in a non-suffix rule is a GNU make idiom.
 - Use $(@D) over $(dir $@). The latter is a POSIX standard.

v2: Cosmetic tweaks in the commit summary.

Cc: 11.0 <59f39c0db42d4479a46b02d4d2bc11120e37bb44@lists.freedesktop.org>
Signed-off-by: Emil Velikov <4c996f2f093ddd0585a556460b4cc3ba555546ef@gmail.com>
Reviewed-by: Matt Turner <789b315743a28dd066a4ba1459c35951c291d8a6@gmail.com> (v1)

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler