cmake: use relative path by default

"${CMAKE_INSTALL_PREFIX}/" is used only when the path should be
absolute path.

CMakeLists.txt

@@ -44,7 +44,7 @@ set(LIB_DIR "lib")
 set(INCLUDE_DIR "include")
 set(GRN_INCLUDE_DIR "include/groonga")
 set(DATA_DIR "share")
-set(GRN_DATA_DIR "${CMAKE_INSTALL_PREFIX}/${DATA_DIR}/${GRN_PROJECT_NAME}")
+set(GRN_DATA_DIR "${DATA_DIR}/${GRN_PROJECT_NAME}")
 set(CONFIG_DIR "etc")
 set(GRN_CONFIG_DIR "${CMAKE_INSTALL_PREFIX}/${CONFIG_DIR}/${GRN_PROJECT_NAME}")
 
@@ -61,7 +61,7 @@ set(GRN_DEFAULT_RELATIVE_DOCUMENT_ROOT
   "share/${GRN_PROJECT_NAME}/${GRN_DEFAULT_DOCUMENT_ROOT_BASE}"
   CACHE PATH "groonga default relative document root")
 set(GRN_DEFAULT_DOCUMENT_ROOT
-  "${GRN_DATA_DIR}/${GRN_DEFAULT_DOCUMENT_ROOT_BASE}"
+  "${CMAKE_INSTALL_PREFIX}/${GRN_DATA_DIR}/${GRN_DEFAULT_DOCUMENT_ROOT_BASE}"
   CACHE PATH "groonga default document root")
 set(GRN_STACK_SIZE
   1024
@@ -76,7 +76,7 @@ set(GRN_DLL_FILENAME
   "${CMAKE_SHARED_LIBRARY_PREFIX}groonga${CMAKE_SHARED_LIBRARY_SUFFIX}")
 set(GRN_QUERY_EXPANDER_TSV_RELATIVE_SYNONYMS_FILE "synonyms.tsv")
 set(GRN_QUERY_EXPANDER_TSV_SYNONYMS_FILE
-  "${GRN_DATA_DIR}/${GRN_QUERY_EXPANDER_TSV_RELATIVE_SYNONYMS_FILE}")
+  "${CMAKE_INSTALL_PREFIX}/${GRN_DATA_DIR}/${GRN_QUERY_EXPANDER_TSV_RELATIVE_SYNONYMS_FILE}")
 
 if(CMAKE_COMPILER_IS_GNUCXX)
   set(GRN_NO_WARNING_COMPILE_FLAGS