Quoting the POSIX standard for the `-D` C compiler flag:

  -D  name[=value]

      Define name as if by a C-language #define directive.
      If no `=value` is given, a value of 1 shall be used.
      [...]

Removing this explicit `=1` definition from `-D` flags which are used to
define boolean macro constants makes the CLI invocation a bit more
idiomatic and reduces visual noise in debugging output
(flags containing `=`s are additionally put in single-quotes by GHC)