llcppgcfg:support generate libs config without pkgconfig search

[luoliwoshang]

like this library https://github.com/FreeRTOS/FreeRTOS-Kernel , doesnt have pc file,if i want generate this config with correct includes field ,i need create a .pc file,it's so complex.

├── include
│   ├── atomic.h
│   ├── CMakeLists.txt
│   ├── croutine.h
│   ├── deprecated_definitions.h
│   ├── event_groups.h
│   ├── FreeRTOS.h
│   ├── list.h
│   ├── message_buffer.h
│   ├── mpu_prototypes.h
│   ├── mpu_syscall_numbers.h
│   ├── mpu_wrappers.h
│   ├── newlib-freertos.h
│   ├── picolibc-freertos.h
│   ├── portable.h
│   ├── projdefs.h
│   ├── queue.h
│   ├── semphr.h
│   ├── stack_macros.h
│   ├── StackMacros.h
│   ├── stdint.readme
│   ├── stream_buffer.h
│   ├── task.h
│   └── timers.h

so i think llcppcfg need support a mode to support these libraries which is have not pc file.