libindi.pc doesn't provide any -l in Libs

[DarthGandalf]

Describe the bug
The purpose of pkg-config is to tell build system of the client of the library what flags to pass to compiler in order to use the library. However, libindi.pc's Libs simply says Libs: -L${libdir}, resulting in undefined symbols in linker stage, unless the client also explicitly provides -lindiclient or such.

To Reproduce
Exact steps to reproduce the behavior.

1. Run pkg-config --libs libindi
2. See empty output

Expected behavior
It should return the list of libraries to link to.

Screenshots
If applicable, add screenshots to help explain your problem.

Desktop (please complete the following information):

• OS: Gentoo Linux
• Version 2.1.1

In https://github.com/indilib/indi/blob/master/libindi.pc.cmake the libraries are supposed to come from @PKG_CONFIG_LIBS@, but cmake never sets this variable