build: use literals for AC_CHECK_HEADERS/AC_CHECK_FUNCS

build: use literals for AC_CHECK_HEADERS/AC_CHECK_FUNCS

autoconf 2.72 reports:

    configure.ac:188: warning: AC_CHECK_HEADERS(\
    ): you should use literals
    ../autoconf-2.71/lib/autoconf/headers.m4:217: AC_CHECK_HEADERS is expanded from...
    configure.ac:188: the top level
    configure.ac:188: warning: AC_CHECK_HEADERS(afunix.h
    ): you should use literals

And also it fails to find fcntl.h and other headers because of this:

    configure:14725: checking for  fcntl.h
    configure:14725: gcc -c -I/mingw64/include  conftest.c >&5
    conftest.c:77:10: fatal error:  fcntl.h: No such file or directory
       77 | #include < fcntl.h>

Refs: https://github.com/libevent/libevent/pull/1146/checks?check_run_id=2211752215