Fix up xmlsec1-config file

[imirkin]

It's effectively a pkg-config file, but not exactly. This is required to install dm.xmlsec.binding 1.3.7 without additional changes.

The source file normally contains, e.g.,

prefix="/usr"
package="xmlsec1"
exec_prefix="${prefix}"
exec_prefix_set=no
libdir="/usr/lib64"

And this command fixes up the prefix and libdir. Tested to successfully install the python-saml package (which depends on dm.xmlsec.binding) with this change. Previously there was an error finding xmlsec/crypto.h due to incorrect include path.

I am loathe to add 1000 fixups like this, but I also don't see a substantially better way. Hopefully the number of such non-standard config files globally will be limited given pkg-config's popularity.