Restore ?= for setting PG_CONFIG.

And add a note about how it allows an environment variable to be used.

doc/howto.md

@@ -146,7 +146,7 @@ The [`pair`](http://github.com/theory/kv-pair/) and [`semver`](http://github.com
     REGRESS_OPTS = --inputdir=test
     DOCS         = $(wildcard doc/*.md)
     # MODULES    = $(patsubst %.c,%,$(wildcard src/*.c))
-    PG_CONFIG    = pg_config
+    PG_CONFIG   ?= pg_config
     PG91         = $(shell $(PG_CONFIG) --version | grep -qE " 8\.| 9\.0" && echo no || echo yes)
 
     ifeq ($(PG91),yes)
@@ -178,6 +178,10 @@ Next we have the `PG_CONFIG` variable. This points to the [`pg_config`](http://w
 
     make PG_CONFIG=/path/to/pg_config
 
+Thanks to the `?=` operator, it can also be set as an environment variable, which is useful for executing multiple `make` commands in a one-liner:
+
+    env PG_CONFIG=/path/to/pg_config make && make installcheck && make install
+
 The `Makefile` next uses `pg_config` to determine whether the extension is being built against PostgreSQL 9.1 or higher. Based on what it finds, extra steps are taken in the following section. That is, if this line returns true:
 
     ifeq ($(PG91),yes)