m4: Add Since releases and cleanup comments

Note which release each macro was added in and cleanup the comments to
be more consistent between them.

pkg.m4.in

@@ -23,7 +23,7 @@
 # the same distribution terms that you use for the rest of that program.
 
 # PKG_PREREQ(MIN-VERSION)
-# -------------------------------------
+# -----------------------
 # Since: 0.29
 #
 # Verify that the version of the pkg-config macros are at least
@@ -45,6 +45,13 @@ m4_if(m4_version_compare(PKG_MACROS_VERSION, [$1]), -1,
 
 # PKG_PROG_PKG_CONFIG([MIN-VERSION])
 # ----------------------------------
+# Since: 0.16
+#
+# Search for the pkg-config tool and set the PKG_CONFIG variable to
+# first found in the path. Checks that the version of pkg-config found
+# is at least MIN-VERSION. If MIN-VERSION is not specified, 0.9.0 is
+# used since that's the first version where most current features of
+# pkg-config existed.
 AC_DEFUN([PKG_PROG_PKG_CONFIG],
 [m4_pattern_forbid([^_?PKG_[A-Z_]+$])
 m4_pattern_allow([^PKG_CONFIG(_(PATH|LIBDIR|SYSROOT_DIR|ALLOW_SYSTEM_(CFLAGS|LIBS)))?$])
@@ -69,6 +76,8 @@ fi[]dnl
 ])# PKG_PROG_PKG_CONFIG
 
 # PKG_CHECK_EXISTS(MODULES, [ACTION-IF-FOUND], [ACTION-IF-NOT-FOUND])
+# -------------------------------------------------------------------
+# Since: 0.18
 #
 # Check to see whether a particular set of modules exists.  Similar
 # to PKG_CHECK_MODULES(), but does not set variables or print errors.
@@ -77,7 +86,6 @@ fi[]dnl
 # only at the first occurence in configure.ac, so if the first place
 # it's called might be skipped (such as if it is within an "if", you
 # have to call PKG_CHECK_EXISTS manually
-# --------------------------------------------------------------
 AC_DEFUN([PKG_CHECK_EXISTS],
 [AC_REQUIRE([PKG_PROG_PKG_CONFIG])dnl
 if test -n "$PKG_CONFIG" && \
@@ -89,6 +97,8 @@ fi])
 
 # _PKG_CONFIG([VARIABLE], [COMMAND], [MODULES])
 # ---------------------------------------------
+# Internal wrapper calling pkg-config via PKG_CONFIG and setting
+# pkg_failed based on the result.
 m4_define([_PKG_CONFIG],
 [if test -n "$$1"; then
     pkg_cv_[]$1="$$1"
@@ -103,7 +113,8 @@ fi[]dnl
 ])# _PKG_CONFIG
 
 # _PKG_SHORT_ERRORS_SUPPORTED
-# -----------------------------
+# ---------------------------
+# Internal check to see if pkg-config supports short errors.
 AC_DEFUN([_PKG_SHORT_ERRORS_SUPPORTED],
 [AC_REQUIRE([PKG_PROG_PKG_CONFIG])
 if $PKG_CONFIG --atleast-pkgconfig-version 0.20; then
@@ -115,15 +126,13 @@ fi[]dnl
 
 
 # PKG_CHECK_MODULES(VARIABLE-PREFIX, MODULES, [ACTION-IF-FOUND],
-# [ACTION-IF-NOT-FOUND])
-#
+#   [ACTION-IF-NOT-FOUND])
+# --------------------------------------------------------------
+# Since: 0.4.0
 #
 # Note that if there is a possibility the first call to
 # PKG_CHECK_MODULES might not happen, you should be sure to include an
 # explicit call to PKG_PROG_PKG_CONFIG in your configure.ac
-#
-#
-# --------------------------------------------------------------
 AC_DEFUN([PKG_CHECK_MODULES],
 [AC_REQUIRE([PKG_PROG_PKG_CONFIG])dnl
 AC_ARG_VAR([$1][_CFLAGS], [C compiler flags for $1, overriding pkg-config])dnl
@@ -181,8 +190,10 @@ fi[]dnl
 
 
 # PKG_CHECK_MODULES_STATIC(VARIABLE-PREFIX, MODULES, [ACTION-IF-FOUND],
-# [ACTION-IF-NOT-FOUND])
+#   [ACTION-IF-NOT-FOUND])
 # ---------------------------------------------------------------------
+# Since: 0.29
+#
 # Checks for existence of MODULES and gathers its build flags with
 # static libraries enabled. Sets VARIABLE-PREFIX_CFLAGS from --cflags
 # and VARIABLE-PREFIX_LIBS from --libs.
@@ -199,8 +210,10 @@ PKG_CONFIG=$_save_PKG_CONFIG[]dnl
 ])
 
 
-# PKG_INSTALLDIR(DIRECTORY)
+# PKG_INSTALLDIR([DIRECTORY])
 # -------------------------
+# Since: 0.27
+#
 # Substitutes the variable pkgconfigdir as the location where a module
 # should install pkg-config .pc files. By default the directory is
 # $libdir/pkgconfig, but the default can be changed by passing
@@ -219,8 +232,10 @@ m4_popdef([pkg_description])
 ]) dnl PKG_INSTALLDIR
 
 
-# PKG_NOARCH_INSTALLDIR(DIRECTORY)
-# -------------------------
+# PKG_NOARCH_INSTALLDIR([DIRECTORY])
+# --------------------------------
+# Since: 0.27
+#
 # Substitutes the variable noarch_pkgconfigdir as the location where a
 # module should install arch-independent pkg-config .pc files. By
 # default the directory is $datadir/pkgconfig, but the default can be
@@ -242,6 +257,8 @@ m4_popdef([pkg_description])
 # PKG_CHECK_VAR(VARIABLE, MODULE, CONFIG-VARIABLE,
 # [ACTION-IF-FOUND], [ACTION-IF-NOT-FOUND])
 # -------------------------------------------
+# Since: 0.28
+#
 # Retrieves the value of the pkg-config variable for the given module.
 AC_DEFUN([PKG_CHECK_VAR],
 [AC_REQUIRE([PKG_PROG_PKG_CONFIG])dnl