FindImageMagick: Find arch include dir (#15007)
FindImageMagick: Provide per-component include dirs (#15007)

Set the ImageMagick_<component>_INCLUDE_DIRS variable that is mentioned
in the documentation but was not set before.
cmGlobalGenerator: Do not re-add CMAKE_<LANG>_COMPILER

The cmGlobalGenerator::ResolveLanguageCompiler method, invoked only by
Makefile generators, contains code originally added by commit v2.4.0~796
(..., add new generators for msys and mingw, 2005-12-22) to compute the
full path to the compiler and save the result back into the cache value.
Since then the CMakeDetermine*Compiler modules have learned to resolve
the full path to the compiler and save it in CMake*Compiler.cmake files
configured in the build tree.  The value of CMAKE_<LANG>_COMPILER in the
cache is now only for reference of what the user originally specified.
The full path is now available in a normal variable of the same name,
and this is used by project code and the generators.

When the user specifies -DCMAKE_<LANG>_COMPILER=name on the command-line
of an existing build tree that uses a Makefile generator, it is first
stored in the cache with an uninitialized type.  Then later when
ResolveLanguageCompiler updates the cache entry and sets the type to
FILEPATH, cmMakefile::AddCacheDefinition does CollapseFullPath on the
"name" and ends up with something like "$PWD/name" which is unlikely to
be correct.  Furthermore, cmMakefile::AddCacheDefinition proceeds to
remove the normal variable of the same name, so the value originally
saved in CMake<LANG>Compiler.cmake is ignored and the generators use the
wrong path to the compiler.

Resolve this by dropping the code from ResolveLanguageCompiler that
touches the cache value of CMAKE_<LANG>_COMPILER.  As explained above it
is no longer needed anyway.
CMakeDetermineCompiler: Simplify CMAKE_<LANG>_COMPILER default force-cache

If find_program does not find CMAKE_<LANG>_COMPILER, use set_property()
to force the value to be that of CMAKE_<LANG>_COMPILER_INIT instead of
set().  This allows us to set the value without re-specifying the type
and documentation, thus preserving what find_program set.
Merge topic 'fix-re-cmake-with-compiler' into next

8981513a CMakeDetermineCompiler: Simplify CMAKE_<LANG>_COMPILER default force-cache
731427a6 cmGlobalGenerator: Do not re-add CMAKE_<LANG>_COMPILER
Merge topic 'FindImageMagick-arch-include-dir' into next

1342e022 FindImageMagick: Provide per-component include dirs (#15007)
6f5581c1 FindImageMagick: Find arch include dir (#15007)