CMake and pkg-config boilerplate

[chillenb]

Adds the necessary mumbo-jumbo to install CMake files for libcint, for use with find_package, as well as a pkg-config file.
I am truly sorry that in 2026 this still takes 50 lines of nonsense.

[chillenb]

I also added the line

target_include_directories(cint
   PUBLIC 
   $<BUILD_INTERFACE:${PROJECT_BINARY_DIR}/include>
   $<INSTALL_INTERFACE:${CMAKE_INSTALL_INCLUDEDIR}>
+  INTERFACE
+  $<BUILD_INTERFACE:${PROJECT_SOURCE_DIR}/include>
   PRIVATE
   ${PROJECT_BINARY_DIR}/src
   ${PROJECT_SOURCE_DIR}/src

so that you can include cint_funcs.h if you are getting libcint with FetchContent or add_subdirectory.