Fix xmlsec detection when cross-compiling with pkg-config

[chewi]

The xmlsec1-config script does not help when cross-compiling. pkg-config is subsequently used for a version detection anyway.

In that version detection, the $PKG_CONFIG variable should be respected.

[vmwclabot]

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[vmwclabot]

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[vmwclabot]

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[johnwvmw]

This pull request was incorporated and release in open-vm-tools 13.0.0. Closing this PR