Add information about CFLAGS env variable in build from source documentation

[gausby]

I just made vernemq build use Erlang 18 on OS X by doing the following.

1. I checked out the currently open pull-request that resolves the Erlang 18 build issues. Make it build and run on erlang 18 #81
2. I merged the current master into this branch, as it resolves an issue with Cuttlefish
3. I ran make rel, but it failed at vmq-passwd, as it couldn't find the openssl headers on my system
4. I fixed this by adding -L/usr/local/opt/openssl/lib -I/usr/local/opt/openssl/include to the Makefile in vmq-passwd/c_src and run a build in that directory.
5. Then I ran the make rel again and it made it all the way through.

I later found out that I could specify the headers using CFLAGS. I think that CFLAGS would be worthy of a mention in the build from source instructions.

[gausby]

I forgot to mention; I have openssl installed via Homebrew. A package manager for OS X.

[larshesel]

Hey @gausby sorry for letting this issue stew for so long, thank you for contributing it. Maybe @ioolkos can adapt the docs to include something about this as he actually owns a mac. If not I'll try and write something up, but full disclosure: I'm no mac user so I'm hardly qualified :)

[dergraf]

closing this issue in favor of #119

[syberkitten]

@gausby This works for me as well, adding this line into the MakeFile:
-L/usr/local/opt/openssl/lib -I/usr/local/opt/openssl/include

final result:

CC?=	gcc

compile:
	$(CC) $(CFLAGS) vmq_passwd.c -L/usr/local/opt/openssl/lib -I/usr/local/opt/openssl/include -lcrypto -o ../priv/vmq_passwd

Now the code compiles on El-Capitan using Erlang OTP/19.3 (with Kerl)

[gausby]

@syberkitten I got it running now, thanks :)