Rename local 'dir_max' variables to 'dir'

The code is not computing the maximum length directory, as is the
case in cmLocalVisualStudioGenerator::ComputeLongestObjectDirectory.
cmGeneratorExpression: Remove unused include.
Makefiles: Remove duplicate code.

The cmGlobalUnixMakefileGenerator3::ProgressMapCompare struct is
logically equivalent to cmStrictTargetComparison.
cmMakefile: Fix typo in comment.
cmMakefile: Fix style.

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Merge topic 'minor_cleanups' into next

19012b46 cmMakefile: Fix style.
10e6f1e7 cmMakefile: Fix typo in comment.
c3a2f78b Makefiles: Remove duplicate code.
2e0611f9 cmGeneratorExpression: Remove unused include.
d83ef53a Rename local 'dir_max' variables to 'dir'