Support absolute path for CMAKE_INSTALL_*DIR

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.

CMakeLists.txt

@@ -30,8 +30,16 @@ LIST (APPEND CMAKE_MODULE_PATH "${PROJECT_SOURCE_DIR}/cmake/")
 include(GNUInstallDirs)
 SET (prefix ${CMAKE_INSTALL_PREFIX})
 SET (exec_prefix ${CMAKE_INSTALL_PREFIX})
-SET (libdir ${CMAKE_INSTALL_LIBDIR})
-SET (includedir ${CMAKE_INSTALL_INCLUDEDIR})
+IF (IS_ABSOLUTE ${CMAKE_INSTALL_LIBDIR})
+    SET (libdir ${CMAKE_INSTALL_LIBDIR})
+ELSE ()
+    SET (libdir "\${prefix}/${CMAKE_INSTALL_LIBDIR}")
+ENDIF ()
+IF (IS_ABSOLUTE ${CMAKE_INSTALL_INCLUDEDIR})
+    SET (includedir ${CMAKE_INSTALL_INCLUDEDIR})
+ELSE ()
+    SET (includedir "\${prefix}/${CMAKE_INSTALL_INCLUDEDIR}")
+ENDIF ()
 
 OPTION (MSGPACK_32BIT "32bit compile" OFF)
 

msgpack-c.pc.in

@@ -1,7 +1,7 @@
 prefix=@prefix@
 exec_prefix=@exec_prefix@
-libdir=${prefix}/@libdir@
-includedir=${prefix}/@includedir@
+libdir=@libdir@
+includedir=@includedir@
 
 Name: MessagePack
 Description: Binary-based efficient object serialization library