cmake: fix pkgconfig and cmake interaction #204

Job	Run time
Make (gcc)	1m 31s
Make (gcc, USE_STANDARD_TMPFILE=1)	1m 36s
Make (gcc, USE_SYSTEM_MINIZIP=1)	1m 59s
Make (gcc, USE_DTOA_LIBRARY=1)	1m 35s
Make (gcc, USE_NO_MD5=1)	1m 31s
Make (gcc, USE_OPENSSL_MD5=1)	1m 42s
Make (gcc, USE_MEM_FILE=1)	1m 32s
Make (clang)	1m 41s
Make (clang, USE_STANDARD_TMPFILE=1)	1m 50s
Make (clang, USE_SYSTEM_MINIZIP=1)	2m 50s
Make (clang, USE_DTOA_LIBRARY=1)	1m 44s
Make (clang, USE_NO_MD5=1)	2m 54s
Make (clang, USE_OPENSSL_MD5=1)	1m 55s
Make (clang, USE_MEM_FILE=1)	1m 37s
	25m 57s

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