Support custom pkg-config executable names

When cross-compiling, it's common to have executables prefixed with
the name of the architecture you're building for,
e.g. aarch64-unknown-linux-musl-cc or x86_64-unknown-freebsd-pkg-config.
Lots of build tools support a PKG_CONFIG environment variable to
enable this use case.

With this change, I was able to successfully cross-compile and run
catgirl.

configure

@@ -1,6 +1,8 @@
 #!/bin/sh
 set -eu
 
+: ${PKG_CONFIG:=pkg-config}
+
 cflags() {
 	echo "CFLAGS += $*"
 }
@@ -8,16 +10,16 @@ defstr() {
 	cflags "-D'$1=\"$2\"'"
 }
 defvar() {
-	defstr "$1" "$(pkg-config --variable=$3 $2)${4:-}"
+	defstr "$1" "$(${PKG_CONFIG} --variable=$3 $2)${4:-}"
 }
 ldadd() {
 	lib=$1; shift
 	echo "LDADD.${lib} = $*"
 }
 config() {
-	pkg-config --print-errors "$@"
-	cflags $(pkg-config --cflags "$@")
-	for lib; do ldadd $lib $(pkg-config --libs $lib); done
+	${PKG_CONFIG} --print-errors "$@"
+	cflags $(${PKG_CONFIG} --cflags "$@")
+	for lib; do ldadd $lib $(${PKG_CONFIG} --libs $lib); done
 }
 
 exec >config.mk