Include PKG_CHECK_VAR macro for reading variables in .pc files

Freedesktop #48098 (https://bugs.freedesktop.org/show_bug.cgi?id=48098)

pkg-config.1

@@ -362,6 +362,13 @@ module should install arch-independent pkg-config .pc files. By default
 the directory is $datadir/pkgconfig, but the default can be changed by
 passing DIRECTORY. The user can override through the
 --with-noarch-pkgconfigdir parameter.
+.TP
+.I "PKG_CHECK_VAR(VARIABLE, MODULE, CONFIG-VARIABLE, [ACTION-IF-FOUND], [ACTION-IF-NOT-FOUND])"
+
+Retrieves the value of the pkg-config variable CONFIG-VARIABLE from
+MODULE and stores it in VARIABLE. Note that repeated usage of VARIABLE
+is not recommended as the check will be skipped if the variable is
+already set.
 
 .SH METADATA FILE SYNTAX
 To add a library to the set of packages \fIpkg-config\fP knows about,

pkg.m4

@@ -197,3 +197,18 @@ AC_SUBST([noarch_pkgconfigdir], [$with_noarch_pkgconfigdir])
 m4_popdef([pkg_default])
 m4_popdef([pkg_description])
 ]) dnl PKG_NOARCH_INSTALLDIR
+
+
+# PKG_CHECK_VAR(VARIABLE, MODULE, CONFIG-VARIABLE,
+# [ACTION-IF-FOUND], [ACTION-IF-NOT-FOUND])
+# -------------------------------------------
+# Retrieves the value of the pkg-config variable for the given module.
+AC_DEFUN([PKG_CHECK_VAR],
+[AC_REQUIRE([PKG_PROG_PKG_CONFIG])dnl
+AC_ARG_VAR([$1], [value of $3 for $2, overriding pkg-config])dnl
+
+_PKG_CONFIG([$1], [variable="][$3]["], [$2])
+AS_VAR_COPY([$1], [pkg_cv_][$1])
+
+AS_VAR_IF([$1], [""], [$5], [$4])dnl
+])# PKG_CHECK_VAR