llvmpipe: re-enable other gallium drivers

No more symbol collisions between drivers, AFAICT.
diff --git a/configs/linux-llvm b/configs/linux-llvm
index e7cf791..5358bf5 100644
--- a/configs/linux-llvm
+++ b/configs/linux-llvm
@@ -8,8 +8,8 @@
 
 #GALLIUM_AUXILIARY_DIRS += gallivm
 
-# override for LLVM
-GALLIUM_DRIVERS_DIRS = llvmpipe
+# Add llvmpipe driver
+GALLIUM_DRIVERS_DIRS += llvmpipe
 
 OPT_FLAGS = -g -ansi -pedantic
 DEFINES += -DDEBUG -DDEBUG_MATH -DGALLIUM_LLVMPIPE