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

15d87d5 cmTarget: Trivially make more API const.
b75203b cmTarget: Make GetExportMacro const.
6e99d5f cmTarget: Make NameResolvesToFramework const.
203089c cmGlobalGenerator: Make NameResolvesToFramework const.
47aa7e8 cmMakefile: Make FindTarget const.
8138dd4 cmMakefile: Make IsAlias const.
ccdf8bb cmTarget: Make GetInterfaceLinkLibraries const.
894415e cmTarget: Make GetTargetSourceFileFlags const.
11cccb3 cmTarget: Make custom command accessors API const.
0f87643 cmGeneratorTarget: Make GetIncludeDirectories const.