Link error caused by incorrect 'Libs' entry in mlpack.pc file

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#### Issue description



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The `Libs` entry of mlpack.pc  seems to be incorrect, which causes the link error.



#### Your environment



 * version of mlpack: 3.2.2

 * operating system:macOS 10.14.6

 * compiler:clang++

 * version of dependencies (Boost/Armadillo):Boost 1.71, Armadillo 9.800

 * any other environment information you think is relevant:

         clang version 9.0.0 (tags/RELEASE_900/final)

         Target: x86_64-apple-darwin18.7.0

#### Steps to reproduce



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possible! -->

Compile the following gist ( compile command is located at the first line )

[test.cpp](https://gist.github.com/tomjpsun/276b6c44046154c7ff11eb17b4c7dded)

#### Expected behavior



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Normally, the sample code should be built without problem.



#### Actual behavior



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We get the build error message:

ld: library not found for -l/usr/local/lib/libarmadillo.dylib

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