osmesa: Allow building standalone in all three channel widths

autoconf had been designating the 8 bit libOSMesa as the default
standalone osmesa, but the Makefile expected it to be linked to libGL.
Fix up the osmesa Makefile so that it allows any of the combinations of
standalone and channel width to be built.

Fixes bug #21980.

src/mesa/drivers/osmesa/Makefile

@@ -19,11 +19,12 @@ INCLUDE_DIRS = \
 	-I$(TOP)/src/mesa \
 	-I$(TOP)/src/mesa/main
 
+# Standalone osmesa needs to be linked with core Mesa APIs
+ifeq ($(DRIVER_DIRS), osmesa)
 CORE_MESA = $(TOP)/src/mesa/libmesa.a $(TOP)/src/mesa/libglapi.a
-
-
-.PHONY: osmesa8
-.PHONY: osmesa16
+else
+CORE_MESA =
+endif
 
 
 .c.o:
@@ -31,31 +32,12 @@ CORE_MESA = $(TOP)/src/mesa/libmesa.a $(TOP)/src/mesa/libglapi.a
 
 
 default: $(TOP)/$(LIB_DIR)/$(OSMESA_LIB_NAME)
-	@ if [ "${DRIVER_DIRS}" = "osmesa" ] ; then \
-		$(MAKE) osmesa16 ; \
-	else \
-		$(MAKE) osmesa8 ; \
-	fi
 
 
-
-
-# The normal libOSMesa is used in conjuction with libGL
-osmesa8: $(TOP)/$(LIB_DIR)/$(OSMESA_LIB_NAME)
-
-$(TOP)/$(LIB_DIR)/$(OSMESA_LIB_NAME): $(OBJECTS)
-	$(MKLIB) -o $(OSMESA_LIB) -linker '$(CC)' -ldflags '$(LDFLAGS)' \
-		-major $(MESA_MAJOR) -minor $(MESA_MINOR) -patch $(MESA_TINY) \
-		-install $(TOP)/$(LIB_DIR) $(MKLIB_OPTIONS) \
-		-id $(INSTALL_LIB_DIR)/lib$(OSMESA_LIB).$(MESA_MAJOR).dylib \
-		$(OSMESA_LIB_DEPS) $(OBJECTS)
-
-
-
-
-# The libOSMesa16/libOSMesa32 libraries do not use libGL but rather are built
-# with all the other Mesa sources (compiled with -DCHAN_BITS=16/32
-osmesa16: $(OBJECTS) $(CORE_MESA)
+# libOSMesa can be used in conjuction with libGL or with all other Mesa
+# sources. We can also build libOSMesa16/libOSMesa32 by setting
+# -DCHAN_BITS=16/32.
+$(TOP)/$(LIB_DIR)/$(OSMESA_LIB_NAME): $(OBJECTS) $(CORE_MESA)
 	$(MKLIB) -o $(OSMESA_LIB) -linker '$(CC)' -ldflags '$(LDFLAGS)' \
 		-major $(MESA_MAJOR) -minor $(MESA_MINOR) -patch $(MESA_TINY) \
 		-install $(TOP)/$(LIB_DIR) $(MKLIB_OPTIONS) \