Vendored build fails on freebsd target

Originally opened this at sfackler/rust-openssl#1307 where I was told to open it here.

When attempting to use the `--feature openssl/vendored` flag on target `x86_64-unknown-freebsd`, I get a panic during compilation. Some relevant logs:
```
... (lots of output that seems irrelevant) ...

cc -I. -Iinclude -fPIC -pthread -Wa,--noexecstack -Wall -O3 -O2 -ffunction-sections -fdata-sections -fPIC -g -fno-omit-frame-pointer -m64 -DL_ENDIAN -DOPENSSL_PIC -DOPENSSL_CPUID_OBJ -DOPENSSL_IA32_SSE2 -DOPENSSL_BN_ASM_MONT -DOPENSSL_BN_ASM_MONT5 -DOPENSSL_BN_ASM_GF2m -DSHA1_ASM -DSHA256_ASM -DSHA512_ASM -DKECCAK1600_ASM -DRC4_ASM -DMD5_ASM -DAESNI_ASM -DVPAES_ASM -DGHASH_ASM -DECP_NISTZ256_ASM -DX25519_ASM -DPOLY1305_ASM -DOPENSSLDIR="\"/home/travis/build/samtay/so/target/x86_64-unknown-freebsd/debug/build/openssl-sys-5fa0352f1ccf2a9f/out/openssl-build/install/ssl\"" -DENGINESDIR="\"/home/travis/build/samtay/so/target/x86_64-unknown-freebsd/debug/build/openssl-sys-5fa0352f1ccf2a9f/out/openssl-build/install/lib/engines-1.1\"" -D_THREAD_SAFE -D_REENTRANT -DNDEBUG -MMD -MF crypto/engine/eng_devcrypto.d.tmp -MT crypto/engine/eng_devcrypto.o -c -o crypto/engine/eng_devcrypto.o crypto/engine/eng_devcrypto.c

make[1]: Leaving directory `/home/travis/build/samtay/so/target/x86_64-unknown-freebsd/debug/build/openssl-sys-5fa0352f1ccf2a9f/out/openssl-build/build/src'

--- stderr

crypto/engine/eng_devcrypto.c:23:30: fatal error: crypto/cryptodev.h: No such file or directory

#include

^

compilation terminated.

make[1]: *** [crypto/engine/eng_devcrypto.o] Error 1

make: *** [build_libs] Error 2

thread 'main' panicked at '

Error building OpenSSL:

Command: "make" "build_libs"

Exit status: exit code: 2

', /home/travis/.cargo/registry/src/github.com-1ecc6299db9ec823/openssl-src-111.10.0+1.1.1g
```
Full output can be seen in the [job log](https://travis-ci.org/github/samtay/so/jobs/701888738).