Issue

When `CMAKE_INSTALL_LIBDIR` and `CMAKE_INSTALL_INCLUDEDIR` are set to absolute paths, the `msgpack-c.pc` file generated by CMake improperly configures `libdir` and `includedir`. This leads to incorrect paths that prevent the compiler from locating necessary header and library files.

How to reproduce

Build and install `msgpack-c`.

```console
% cmake -S . -B ../msgpack-c.build -DCMAKE_INSTALL_LIBDIR=/tmp/local/lib -DCMAKE_INSTALL_INCLUDEDIR=/tmp/local/include
% cmake --build ../msgpack-c.build
% sudo cmake --install ../msgpack-c.build
```

Compile `example/simple_c.c` using installed msgpack-c. The following error happens because the linker cannot find paths provided by pkg-config.

```console
% export PKG_CONFIG_PATH=/tmp/local/lib/pkgconfig:$PKG_CONFIG_PATH
% gcc -o simple_c example/simple_c.c $(pkg-config --cflags --libs msgpack-c)
/usr/bin/ld: cannot find -lmsgpack-c: No such file or directory
collect2: error: ld returned 1 exit status
```

Expected

Successfully compile `example/simple_c.c` using installed msgpack-c. We can execute `simple_c` like the following.

```console
% gcc -o simple_c example/simple_c.c $(pkg-config --cflags --libs msgpack-c)
% ./simple_c
93 01 c3 a7 65 78 61 6d 70 6c 65
[1, true, "example"]
```

Explain the problem in detail

The generated `msgpack-c.pc` file does not handle absolute paths correctly. Here is the result of the incorrect configuration in `How to reproduce` section. In the following `msgpack-c.pc` file, `libdir` and `includedir` are showing unrecognized paths, leading to incorrect paths.

```console
% cat /tmp/local/lib/pkgconfig/msgpack-c.pc
prefix=/usr/local
exec_prefix=/usr/local
libdir=${prefix}//tmp/local/lib <- Here the path is wrong. We expected `/tmp/local/lib`
includedir=${prefix}//tmp/local/include <- Here the path is wrong. We expected `/tmp/local/include`

Name: MessagePack
Description: Binary-based efficient object serialization library
Version: 6.0.1
Libs: -L${libdir} -lmsgpack-c
Cflags: -I${includedir}
```

Solution

Modify the `CMakeLists.txt` file to ensure that `libdir` and `includedir` use absolute paths. This change addresses the issue by providing correct paths to the compiler and linker.