必要なやつをリンクさせる気がするのですが…

$gcc -o main.c md5_crypt.o
/usr/lib/crt1.o: In function `_start':
/usr/lib/crt1.o(.text+0x18): undefined reference to `main'
md5_crypt.o: In function `md5_crypt':
md5_crypt.o(.text+0x45): undefined reference to `MD5Init'
md5_crypt.o(.text+0x64): undefined reference to `MD5Update'
md5_crypt.o(.text+0x83): undefined reference to `MD5Update'
md5_crypt.o(.text+0x93): undefined reference to `MD5Final'
collect2: ld returned 1 exit status