Add pkg-config support.

[ossama-othman]

Generate a `marc.pc' pkg-config file that may be used by MaRC library users to obtain the necessary build related information.

[coveralls]

Pull Request Test Coverage Report for Build 514

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 84.581%

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Totals
Change from base Build 512:	0.0%
Covered Lines:	2622
Relevant Lines:	3100

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💛 - Coveralls