C API header does not compile with a C compiler

The admire.h header does not compile when using a C compiler because of a couple of c++ism:

• The bool type needs stdbool.h to be included
• All references to enum or union types need to include the appropriate keyword (for ex. enum ADM_return_t)
• The type ADM_server is defined as an alias of the incomplete type adm_server, I presume the intention was to use an opaque pointer: typedef struct adm_server* ADM_server_t.

I wasn't planning on creating a merge request for a single file, here's a fix proposal: admire.h