Decrease line length
cmTarget: Make custom command accessors API const.

Add specific mutators instead of providing non-const refs.
cmTarget: Make GetTargetSourceFileFlags const.
cmTarget: Make GetInterfaceLinkLibraries const.
cmMakefile: Make IsAlias const.
cmMakefile: Make FindTarget const.
cmGlobalGenerator: Make NameResolvesToFramework const.
cmTarget: Make NameResolvesToFramework const.
cmTarget: Make GetExportMacro const.

The std::string member is only used for memory management.
cmTarget: Trivially make more API const.
Merge topic 'constify' into next

8318f30 Decrease line length
Merge topic 'constify' into next

15eeace cmTarget: Trivially make more API const.
be9dfb6 cmTarget: Make GetExportMacro const.
0794c13 cmTarget: Make NameResolvesToFramework const.
1c27521 cmGlobalGenerator: Make NameResolvesToFramework const.
37554ac cmMakefile: Make FindTarget const.
8841d73 cmMakefile: Make IsAlias const.
36e31cd cmTarget: Make GetInterfaceLinkLibraries const.
04d398d cmTarget: Make GetTargetSourceFileFlags const.
50d1520 cmTarget: Make custom command accessors API const.