Disable parallel building as parallel builds fail due to incomplete
depends between autogenerated CoreSlave.h and it's include sites:
```
CC prealloc_surface_pool_bridge.lo
prealloc_surface_pool_bridge.c:41:10:
fatal error: core/CoreSlave.h: No such file or directory
```
Dependencies are specified manually in src/core/Makefile.am. Instead
of fixing them one by one locally let's disable parallel builds until
upstream fixes them.
df206b3f00