makefile: fix shell tab-completions (#147)

Shell tab-completions are currently broken due to the regeneration of
`make_config.mk` on every invocation, which also causes a make restart.

Prevent regeneration if the file exists and we're in an invocation that
doesn't execute rule recipes (-n or -q).

Makefile

@@ -85,6 +85,9 @@ endif
 
 $(info $$DEBUG_LEVEL is $(DEBUG_LEVEL), $$LIB_MODE is $(LIB_MODE))
 
+# Only regenerate make_config.mk if it doesn't exists or if we're invoked in a mode
+# that executes target recipes (i.e. not -n or -q)
+ifeq ($(and $(or $(findstring n,$(MAKEFLAGS)),$(findstring q,$(MAKEFLAGS))),$(wildcard make_config.mk)),)
 # Detect what platform we're building on.
 # Export some common variables that might have been passed as Make variables
 # instead of environment variables.
@@ -101,6 +104,7 @@ dummy := $(shell (export ROCKSDB_ROOT="$(CURDIR)"; \
 		  export ROCKSDB_CXX_STANDARD="$(ROCKSDB_CXX_STANDARD)"; \
 		  export USE_FOLLY="$(USE_FOLLY)"; \
                   "$(CURDIR)/build_tools/build_detect_platform" "$(CURDIR)/make_config.mk"))
+endif
 # this file is generated by the previous line to set build flags and sources
 include make_config.mk