Execute pkg-config(1) through shell not make

`VAR != val' in make(1) executes `val' everytime the makefile is parsed,
which a) slows down operations across the whole ports tree and b) may print
warnings from such commands.

`VAR = $$(val)', given VAR ends up in a shell command, will be run during
builds.

Switch to the latter to avoid slowdown/noise for porters, even though this
means that ports like devel/xsd now run the same pkg-config command on
every cc(1) invocation rather than just ones in the makefile.

(Most but not all ports do it this way.)

Pointed out by naddy
OK rsadowski naddy

devel/xsd/Makefile

@@ -45,10 +45,10 @@ MAKE_FLAGS =		AS=${AS:Q} \
 # show compiler lines
 MAKE_FLAGS +=		verbose=1
 
-XERCES_CFLAGS !=	pkg-config --cflags xerces-c
+XERCES_CFLAGS =		`pkg-config --cflags xerces-c`
 CXXFLAGS +=		${XERCES_CFLAGS}
 
-XERCES_LIBS !=		pkg-config --libs xerces-c
+XERCES_LIBS =		`pkg-config --libs xerces-c`
 LDFLAGS +=		${XERCES_LIBS}
 
 # using PREFIX in FAKE_FLAGS is not enough