cmake: generated .pc file contains strange entries

[reddwarf69]

The Libs.private field of the generated .pc file contains -lgcc -lgcc_s -lc -lgcc -lgcc_s.

I guess because of the use of CMAKE_C_IMPLICIT_LINK_LIBRARIES in https://github.com/curl/curl/blob/master/CMakeLists.txt#L1451. Not sure why it's there, the autotools generated file doesn't contain all that.

[reddwarf69]

FWIW I am personally using https://github.com/RedDwarf69/curl/tree/cmake. I'm not creating PRs because I have not tried to make it ready for that (e.g I'm increasing the minimum required cmake version... and I gave no idea how much you want to keep cmake 3.2 support).
I'm solving this with reddwarf69@b620948