MDEV-20596 Configure fails with newer CMake

Fix build error with newer cmake

Fixes the following build error:

CMake Error at cmake/os/Linux.cmake:29 (STRING):
STRING sub-command REPLACE requires at least four arguments.
Call Stack (most recent call first):
CMakeLists.txt:101 (INCLUDE)

CMake Error at cmake/os/Linux.cmake:29 (STRING):
STRING sub-command REPLACE requires at least four arguments.
Call Stack (most recent call first):
CMakeLists.txt:101 (INCLUDE)

The error happens when CMAKE_SHARED_LINKER_{LANG}_FLAGS is not set.
Force the variable to be set to "" as input to prevent this.
Signed-off-by: Ryan Coe <bluemrp9@gmail.com>
Signed-off-by: Vicențiu Ciorbaru <vicentiu@mariadb.org>

cmake/os/Linux.cmake

@@ -26,9 +26,9 @@ SET(CMAKE_REQUIRED_DEFINITIONS ${CMAKE_REQUIRED_DEFINITIONS} -D_GNU_SOURCE=1)
 
 # Fix CMake (< 2.8) flags. -rdynamic exports too many symbols.
 FOREACH(LANG C CXX)
-  STRING(REPLACE "-rdynamic" "" 
+  STRING(REPLACE "-rdynamic" ""
   CMAKE_SHARED_LIBRARY_LINK_${LANG}_FLAGS
-  ${CMAKE_SHARED_LIBRARY_LINK_${LANG}_FLAGS}  
+  "${CMAKE_SHARED_LIBRARY_LINK_${LANG}_FLAGS}"
   )
 ENDFOREACH()