The new PkgConfig template uses PkgConfig.Cmd, by default the
pkg-config executable found on PATH, to check availability of
pkg-config modules.

Each module must have the $(module).AtLeastVersion variable defined, the version
check is checked at compile time but is not enforced. The outcome of the
check is stored in PkgConfig.Modules.$(module).Status and is one of
"sufficient", "outdated" or "missing".

Each module that is not missing will have a number of standard variables
populated under the PkgConfig.Modules.$(module) namespace, those are
.Version, .Cflags, .CflagsOnlyI, .CflagsOnlyOther, .Libs,
.LibsOnlyLowerL, .LibsOnlyL and .LibsOnlyOther.

In addition a module may also have any number of module variable names
listed in $(module).Variables. Those are resolved using pkg-config
--variable= and are available under the namespace
PkgConfig.Modules.$(module).Variables.$(variable).

Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@huawei.com>