cmMakefile: Make GetProperty const.
cmSourceFileLocation: Use a const cmMakefile.
cmSourceFile: Use a const cmMakefile.
cmMakefile: Constify some cmSourceGroup related code.
cmSourceGroup: Fix method name capitalization.

Adhere to the dominant style.
cmCacheManager: Consify version accessors.
cmMakefile: Constify policies accessors.
cmMakefile: Consify custom command API.
cmMakefile: Remove method declarations with no implementation.
cmMakefile: Remove non-const version of method

The const version suffices.
cmMakefile: Trivially constify some methods.
cmMakefile: Make FindSourceGroup const.

Return a pointer instead of a reference.  This allows making the accessor
const with the least impact.
Merge topic 'constify' into next

b3b047ee cmMakefile: Make FindSourceGroup const.
01d7ceda cmMakefile: Trivially constify some methods.
9073318f cmMakefile: Remove non-const version of method
9b05bc4f cmMakefile: Remove method declarations with no implementation.
c6c0bd9d cmMakefile: Consify custom command API.
d6d4eaac cmMakefile: Constify policies accessors.
2a6e56e0 cmCacheManager: Consify version accessors.
5cc9fb02 cmSourceGroup: Fix method name capitalization.
fe8b0330 cmMakefile: Constify some cmSourceGroup related code.
e68a3ead cmSourceFile: Use a const cmMakefile.
b6292402 cmSourceFileLocation: Use a const cmMakefile.
b33ea578 cmMakefile: Make GetProperty const.