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 <mesa-stable@lists.freedesktop.org>
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Matt Turner <mattst88@gmail.com> (v1)

src/util/Makefile.am

@@ -55,5 +55,7 @@ BUILT_SOURCES = $(MESA_UTIL_GENERATED_FILES)
 CLEANFILES = $(BUILT_SOURCES)
 EXTRA_DIST = format_srgb.py SConscript
 
-format_srgb.c: $(srcdir)/format_srgb.py
-	$(AM_V_GEN) $(PYTHON2) $< > $@
+PYTHON_GEN = $(AM_V_GEN)$(PYTHON2) $(PYTHON_FLAGS)
+
+format_srgb.c: format_srgb.py
+	$(PYTHON_GEN) $(srcdir)/format_srgb.py > $@