Use relative install path for pkgconfig files

It is recommended in CMake's official documentation to use relative path
for DESTINATION parameter. Using absolute paths do not work with the
cmake --install command's --prefix option.

CMakeLists.txt

@@ -68,7 +68,7 @@ endif()
 csfml_set_option(CSFML_INSTALL_PKGCONFIG_FILES ${CSFML_INSTALL_PKGCONFIG_DEFAULT} BOOL "TRUE to automatically install pkg-config files so other projects can find SFML")
 
 if(CSFML_INSTALL_PKGCONFIG_FILES)
-    csfml_set_option(CSFML_PKGCONFIG_INSTALL_PREFIX "${CMAKE_INSTALL_PREFIX}/${CSFML_PKGCONFIG_DIR}" PATH "Install directory for CSFML's pkg-config .pc files")
+    csfml_set_option(CSFML_PKGCONFIG_INSTALL_DIR "${CSFML_PKGCONFIG_DIR}" PATH "Install directory for CSFML's pkg-config .pc files")
 
     foreach(csfml_module IN ITEMS all system window graphics audio network)
         configure_file(
@@ -76,7 +76,7 @@ if(CSFML_INSTALL_PKGCONFIG_FILES)
             "tools/pkg-config/csfml-${csfml_module}.pc"
             @ONLY)
         install(FILES "${CMAKE_CURRENT_BINARY_DIR}/tools/pkg-config/csfml-${csfml_module}.pc"
-            DESTINATION "${CSFML_PKGCONFIG_INSTALL_PREFIX}")
+            DESTINATION "${CSFML_PKGCONFIG_INSTALL_DIR}")
     endforeach()
 endif()
 

cmake/Config.cmake

@@ -34,10 +34,10 @@ endif()
 
 # set pkgconfig install directory
 # this could be e.g. macports on mac or msys2 on windows etc.
-set(CSFML_PKGCONFIG_DIR "/${CMAKE_INSTALL_LIBDIR}/pkgconfig")
+set(CSFML_PKGCONFIG_DIR "${CMAKE_INSTALL_LIBDIR}/pkgconfig")
 
 if(SFML_OS_FREEBSD OR SFML_OS_OPENBSD OR SFML_OS_NETBSD)
-    set(CSFML_PKGCONFIG_DIR "/libdata/pkgconfig")
+    set(CSFML_PKGCONFIG_DIR "libdata/pkgconfig")
 endif()
 
 # detect the compiler and its version