gcc - unknown switches (absent also from the manpage)

The option is shown as "-l_library_" (no space) or "-l _library_" (with a space) and c is the library argument,
see https://linux.die.net/man/1/gcc

-lc will link libc (-lfoobar would link libfoobar etc.)

General information about options and arguments

UNIX commands often accept option arguments with or without whitespace. If you have an option o which takes an argument arg you can write -o arg or -oarg. On the other hand you can combine options that don't take an argument, e.g. -a -b -c or -abc.
When you see -lc you can only find out from the documentation (man page) if this is the combination of options -l and -c or option -l with argument c or a single option -lc.

See also https://www.gnu.org/software/libc/manual/html_node/Argument-Syntax.html

Note: gcc is an exception from this general concept. You cannot combine options for gcc.

-lc links with libc, the C library.

See the description of -llibrary in the documentation.