Improper use of PKG_CHECK_MODULES

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First call to PKG_CHECK_MODULES must be unconditional.
If it isn't,  PKG_PROG_PKG_CONFIG should be called first.
Otherwise subsequent PKG_CHECK_MODULES calls (after first
PKG_CHECK_MODULES that was skipped) will not be able to find pkg-config.

I'm hitting this when i configure without explicit --with-glib

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