KNOWN_BUGS: cmake libcurl.pc uses absolute library paths

Closes #6169

docs/KNOWN_BUGS

@@ -128,6 +128,7 @@ problems may have been fixed or changed somewhat since this was written!
  15.5 build on Linux links libcurl to libdl
  15.6 uses -lpthread instead of Threads::Threads
  15.7 generated .pc file contains strange entries
+ 15.8 libcurl.pc uses absolute library paths
 
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@@ -874,3 +875,11 @@ problems may have been fixed or changed somewhat since this was written!
  -lgcc -lgcc_s
 
  See https://github.com/curl/curl/issues/6167
+
+15.8 libcurl.pc uses absolute library paths
+
+ The libcurl.pc file generated by cmake contains things like Libs.private:
+ /usr/lib64/libssl.so /usr/lib64/libcrypto.so /usr/lib64/libz.so. The
+ autotools equivalent would say Libs.private: -lssl -lcrypto -lz
+
+ See https://github.com/curl/curl/issues/6169