cmake: Improve summary output about ccache

[hebasto]

1. From build: Introduce CMake-based build system bitcoin/bitcoin#30454 (comment):

I think we can improve the output when -DWITH_CCACHE=OFF is used. Depending on the system, that output might be:

cmake -B build -DWITH_CCACHE=OFF
< snip >
Use ccache for compiling .............. ccache masquerades as the compiler

We should probably at least indicate that the option was respected by the build-system.

With this PR on Fedora 40:

$ cmake -B build -DWITH_CCACHE=OFF
< snip >
Use ccache for compiling .............. ON


  ******

CMake Warning at CMakeLists.txt:656 (message):
  Disabling ccache was attempted using -DWITH_CCACHE=OFF, but ccache
  masquerades as the compiler.


  ******

2. The second commit fixes a bug introduced in cmake: Implement prefix maps #298.

[TheCharlatan]

lgtm