build: avoid needless re-executions

The USER_UID and DUMPCAP_GROUP variables are recursively assigned, but
contain shell execution. It's not terribly expensive execution, but these
variables are substituted into every compile line, so they're executed
dozens of times, even though their values will never change.

Move them into immediate assignment (which makes DUMPCAP_GROUP a bit more
complex to allow it to be overridden).

This speeds up near-do-nothing runs like 'make help' so that they take only
about 1/10s now, which is acceptably fast.

Signed-off-by: Nick Alcock <nick.alcock@oracle.com>

GNUmakefile

@@ -44,15 +44,18 @@ BPFCFLAGS ?= -O2 -Wall -Wno-unknown-pragmas
 export BPFLD = bpf-unknown-none-ld
 
 # The first non-system uid on this system.
-USER_UID=$(shell grep '^UID_MIN' /etc/login.defs | awk '{print $$2;}')
+USER_UID := $(shell grep '^UID_MIN' /etc/login.defs | awk '{print $$2;}')
 
 # A uid suitable for unprivileged execution.
 UNPRIV_UID ?= -3
 
 # The group one must run as to invoke dumpcap: by default the group of
 # the dumpcap binary.  If dumpcap is owned by root, use the same gid as
 # the UNPRIV_UID unless otherwise overridden.
-DUMPCAP_GROUP ?= $(filter-out root,$(shell stat -c %G /usr/sbin/dumpcap /usr/bin/dumpcap 2>/dev/null | head -1))
+# (We avoid ?= here to avoid rerunning the stat over and over again.)
+ifeq ($(origin DUMPCAP_GROUP), undefined)
+DUMPCAP_GROUP := $(filter-out root,$(shell stat -c %G /usr/sbin/dumpcap /usr/bin/dumpcap 2>/dev/null | head -1))
+endif
 
 # Unwritable but readable directory suitable for overriding as the $HOME of
 # unprivileged processes.