Use double quotes to quote a string with the backslash/newline pair.
Newer gmake versions, such as GNU Make 4.1, do not remove the backslash
from a string quoted with single quotes, which results in an extraneous
backslash passed to the shell and, eventually, to the Go compiler.
Redefinition of typedef is a C11 feature. On older compilers that
default to C99, such as clang 3.4.1 supplied with FreeBSD 10, this
caused -Wtypedef-redefinition warnings.