trivial sync of pkg.m4 from freedesktop

This contains 3 changes, mostly inconsequential:

- fix some spelling issues in the comments

  eb866ade77

- bump the serial number; upstream rationale is very 🤷 but it is
  what it is:
  > There aren't any significant changes here, but this will cause aclocal
  > to get the latest version.

  677e924875

- make the status log for PKG_CHECK_MODULES say "checking for <mod>..."
  instead of "checking for <VAR>..."

  3b96e7434c

There are additional whitespace-only changes that could be synced, and
were explicitly synced in commit 95b683c864
but then undone in commit 360a818f2f so
rather than fighting over it, are simply ignored. Diffing against the
freedesktop version of pkg.m4 will therefore show additional
inconsistencies.

pkg.m4

@@ -1,5 +1,5 @@
-# pkg.m4 - Macros to locate and utilise pkg-config.   -*- Autoconf -*-
-# serial 11 (pkg-config-0.29.1)
+# pkg.m4 - Macros to locate and use pkg-config.   -*- Autoconf -*-
+# serial 12 (pkg-config-0.29.2)
 
 dnl Copyright © 2004 Scott James Remnant <scott@netsplit.com>.
 dnl Copyright © 2012-2015 Dan Nicholson <dbn.lists@gmail.com>
@@ -41,7 +41,7 @@ dnl
 dnl See the "Since" comment for each macro you use to see what version
 dnl of the macros you require.
 m4_defun([PKG_PREREQ],
-[m4_define([PKG_MACROS_VERSION], [0.29.1])
+[m4_define([PKG_MACROS_VERSION], [0.29.2])
 m4_if(m4_version_compare(PKG_MACROS_VERSION, [$1]), -1,
     [m4_fatal([pkg.m4 version $1 or higher is required but ]PKG_MACROS_VERSION[ found])])
 ])dnl PKG_PREREQ
@@ -86,7 +86,7 @@ dnl Check to see whether a particular set of modules exists. Similar to
 dnl PKG_CHECK_MODULES(), but does not set variables or print errors.
 dnl
 dnl Please remember that m4 expands AC_REQUIRE([PKG_PROG_PKG_CONFIG])
-dnl only at the first occurence in configure.ac, so if the first place
+dnl only at the first occurrence in configure.ac, so if the first place
 dnl it's called might be skipped (such as if it is within an "if", you
 dnl have to call PKG_CHECK_EXISTS manually
 AC_DEFUN([PKG_CHECK_EXISTS],
@@ -142,7 +142,7 @@ AC_ARG_VAR([$1][_CFLAGS], [C compiler flags for $1, overriding pkg-config])dnl
 AC_ARG_VAR([$1][_LIBS], [linker flags for $1, overriding pkg-config])dnl
 
 pkg_failed=no
-AC_MSG_CHECKING([for $1])
+AC_MSG_CHECKING([for $2])
 
 _PKG_CONFIG([$1][_CFLAGS], [cflags], [$2])
 _PKG_CONFIG([$1][_LIBS], [libs], [$2])