Print a useful error when pkg-config is missing
Without this check, the PKG_CHECK_MODULES() macro remains unexpanded, which gives a syntax error when the shell reaches that point in the configure script, even for checks which are disabled (e.g., the OpenSSL check when using --disable-http). Instead, explicitly check for pkg-config and give the user useful advice on how to solve the problem if it's not found. Thanks to eXpl0it3r for reporting a problem for which installing pkg-config turned out to be the solution.
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