Any decent C compiler has a warning for the situation that an external function is called without a prototype declaration having been seen.

GCC also has diagnostics for when a function definition occurs without a declaration.

From GCC man page:

       -Wmissing-prototypes (C and Objective-C only)
           Warn if a global function is defined without a previous prototype
           declaration.  This warning is issued even if the definition itself
           provides a prototype.  The aim is to detect global functions that
           fail to be declared in header files.

       -Wstrict-prototypes (C and Objective-C only)
           Warn if a function is declared or defined without specifying the
           argument types.  (An old-style function definition is permitted
           without a warning if preceded by a declaration which specifies the
           argument types.)

       -Wimplicit-function-declaration (C and Objective-C only)
           Give a warning whenever a function is used before being declared.
           In C99 mode (-std=c99 or -std=gnu99), this warning is enabled by
           default and it is made into an error by -pedantic-errors. This
           warning is also enabled by -Wall.