MFH: r521828

Attempt to fix build after removal of Linux c6.

• Replace build dependency on linux-c7-devtools with USE_LINUX=devtools:build so it takes into account LINUX_DEFAULT.
• Install libusb.so.3 which is the SONAME of the library.
• Disable -Werror.
• Don't build profiling library. It's not installed and -pg and -fstack-protector are incompatible.
• Don't let the build make parse /etc/make.conf because this may redefine *FLAGS. The file is already handled by the ports tree.
• Create symbolic links instead of hard links.