Move the safety check about pkg-config in bootstrap.sh.

Don't check twice for pkg-config during configure, since the undefined
macro would be possibly caused on a different system.

bootstrap.sh

@@ -40,6 +40,15 @@ run_versioned() {
 
 set -ex
 
+# We check for this here, because if pkg-config is not found in the
+# system, it's likely that the pkg.m4 macro file is also not present,
+# which will make PKG_PROG_PKG_CONFIG be undefined and the generated
+# configure file faulty.
+if ! pkg-config --version &>/dev/null; then
+    echo "pkg-config is required to bootstrap this program" &>/dev/null
+    exit 1
+fi
+
 if [ "x$1" = "xam" ] ; then
     run_versioned automake "$VERSION" -a -c --foreign
     ./config.status

configure.ac

@@ -462,14 +462,6 @@ AC_SUBST(pulselocaledir)
 
 #### pkg-config ####
 
-# Check for pkg-config manually first, as if its not installed the
-# PKG_PROG_PKG_CONFIG macro won't be defined.
-AC_CHECK_PROG(have_pkg_config, pkg-config, yes, no)
-
-if test x"$have_pkg_config" = "xno"; then
-    AC_MSG_ERROR(pkg-config is required to install this program)
-fi
-
 PKG_PROG_PKG_CONFIG
 
 #### X11 (optional) ####