Issue Description:

I was trying to build swift-nio-ssl on an Ubuntu machine that didn't have libssl-dev installed. The error message was invisible when using swift build but when directly invoking the command line as given by swift build --verbose, I was able to find the problem. This bug report is about the fact that swift build had no build errors, and just quietly failed.

Log output follows.

pseudo@Pseudo:~/swift-nio-ssl$ swift build --verbose

lsb_release -r

pkg-config --variable pc_path pkg-config

uname

'swift-nio-ssl-support' openssl.pc: warning: couldn't find pc file

which clang

/usr/bin/clang -target x86_64-unknown-linux --sysroot / -fPIC -g -O0 -DSWIFT_PACKAGE=1 -DDEBUG=1 -fblocks -fmodules -fmodule-name=CNIOOpenSSL -I /home/pseudo/swift-nio-ssl/Sources/CNIOOpenSSL/include -fmodule-map-file=/home/pseudo/swift-nio-ssl/.build/checkouts/swift-nio-ssl-support/module.modulemap -fmodules-cache-path=/home/pseudo/swift-nio-ssl/.build/x86_64-unknown-linux/debug/ModuleCache -MD -MT dependencies -MF /home/pseudo/swift-nio-ssl/.build/x86_64-unknown-linux/debug/CNIOOpenSSL.build/shims.c.d -c /home/pseudo/swift-nio-ssl/Sources/CNIOOpenSSL/shims.c -o /home/pseudo/swift-nio-ssl/.build/x86_64-unknown-linux/debug/CNIOOpenSSL.build/shims.c.o

:

.

pseudo@Pseudo:~/swift-nio-ssl$ /usr/bin/clang -target x86_64-unknown-linux --sysroot / -fPIC -g -O0 -DSWIFT_PACKAGE=1 -DDEBUG=1 -fblocks -fmodules -fmodule-name=CNIOOpenSSL -I /home/pseudo/swift-nio-ssl/Sources/CNIOOpenSSL/include -fmodule-map-file=/home/pseudo/swift-nio-ssl/.build/checkouts/swift-nio-ssl-support/module.modulemap -fmodules-cache-path=/home/pseudo/swift-nio-ssl/.build/x86_64-unknown-linux/debug/ModuleCache -MD -MT dependencies -MF /home/pseudo/swift-nio-ssl/.build/x86_64-unknown-linux/debug/CNIOOpenSSL.build/shims.c.d -c /home/pseudo/swift-nio-ssl/Sources/CNIOOpenSSL/shims.c -o /home/pseudo/swift-nio-ssl/.build/x86_64-unknown-linux/debug/CNIOOpenSSL.build/shims.c.o

In file included from /home/pseudo/swift-nio-ssl/Sources/CNIOOpenSSL/shims.c:16:

/home/pseudo/swift-nio-ssl/Sources/CNIOOpenSSL/include/c_nio_openssl.h:17:10: fatal error: 'openssl/conf.h' file not found

#include <openssl/conf.h>

     ^~~~~~~~~~~~~~~~

1 error generated.

pseudo@Pseudo:~/swift-nio-ssl$