The curl library should be build in release mode for release versions

**Describe the bug**
The build type is hard coded.

**Environment information:**

- Any

**Steps to Reproduce**
Just look at the code https://github.com/simelo/libskycoin/blob/3cb637dbd49eb3073d18b25205cc58e2519a28c7/lib/curl/CMakeLists.txt#L9

**Actual behavior**
The curl library is build in debug as unique option.

**Expected behavior**
Be able to set a predefined value via command line.

**Additional context**
It's very **important** to move on this if you regenerate the CMakeLists.txt file.