$ cat foo.c
#include <stdio.h>

#ifndef VERSION_2
void foo(int f) {
     printf("version 1 called\n");
}

#else
void foo_v1(int f) {
     printf("version 1 called\n");
}
__asm__(".symver foo_v1,foo@VERSION_1");

void foo_v2(int f) {
     printf("version 2 called\n");
}
/* i.e. foo_v2 is really foo@VERSION_2
 * @@ means this is the default version
 */
__asm__(".symver foo_v2,foo@@VERSION_2");

#endif
$ cat 1.ver
VERSION_1 {
      global:
      foo;
      local:
        *;
};
$ cat 2.ver
VERSION_1 {
      local:
        *;
};

VERSION_2 {
      foo;
} VERSION_1;
$ cat main.c
#include <stdio.h>

void foo(int);

int main(void) {
    foo(100);
    return 0;
}
$ cat Makefile
all: v1 v2

libfoo.so.1 : foo.c
        gcc -shared -fPIC -o libfoo.so.1 -Wl,--soname='libfoo.so.1' -Wl,--version-script=1.ver foo.c

libfoo.so.2 : foo.c
        gcc -shared -fPIC -DVERSION_2 -o libfoo.so.2 -Wl,--soname='libfoo.so.2' -Wl,--version-script=2.ver foo.c

v1: main.c libfoo.so.1
    ln -sf libfoo.so.1 libfoo.so
    gcc -Wall -o v1 -lfoo -L. -Wl,-rpath=. main.c

v2: main.c libfoo.so.2
    ln -sf libfoo.so.2 libfoo.so
    gcc -Wall -o v2 -lfoo -L. -Wl,-rpath=. main.c

.PHONY: clean
clean:
    rm -f libfoo* v1 v2
$ ./v1
version 1 called
$ ./v2
version 2 called