Mroonga cmake failure - LZ4_LIBS = NOTFOUND

The following variables are used in this project, but they are set to NOTFOUND.
LZ4_LIBS

The reason for the failure is that pkg_check_modules will not guarantee
<prefix>_LIBRARY_DIRS variable to be set, according to documentation.
When it's not set, we would force find_library to look in an empty path
and thus fail to correctly find LZ4_LIBS, although pck_check_modules
did previously discover that the library is installed.

To fix the problem and still keep the logic of first following
LIBLZ4_LIBRARY_DIRS and *then* look at other paths, we call find_library
twice. This is the recommended approach, according to CMake 3.11
documentation.

storage/mroonga/vendor/groonga/CMakeLists.txt

@@ -361,10 +361,18 @@ if(NOT ${GRN_WITH_LZ4} STREQUAL "no")
       pkg_check_modules(LIBLZ4 liblz4)
     endif()
     if(LIBLZ4_FOUND)
+      # According to CMake documentation, this is the recommended way to force
+      # looking in LIBRARY_DIRS first and in regular system paths otherwise.
+      #
+      # pkg_check_modules does not guarantee that LIBLZ4_LIBRARY_DIRS will be
+      # set. If it's not set we won't find the library without looking through
+      # the regular system paths.
       find_library(LZ4_LIBS
-	NAMES ${LIBLZ4_LIBRARIES}
-	PATHS ${LIBLZ4_LIBRARY_DIRS}
-	NO_DEFAULT_PATH)
+        NAMES ${LIBLZ4_LIBRARIES}
+        PATHS ${LIBLZ4_LIBRARY_DIRS}
+        NO_DEFAULT_PATH)
+      find_library(LZ4_LIBS
+        NAMES ${LIBLZ4_LIBRARIES})
       set(GRN_WITH_LZ4 TRUE)
     else()
       if(${GRN_WITH_LZ4} STREQUAL "yes")