Add option to link against libtcmalloc.so

Implementation done by Alberto Griggio (agriggio)
for his fork at https://bitbucket.org/agriggio/art

libtcmalloc.so comes with google's perftools:
https://github.com/gperftools/gperftools

This should fix issues where glibc won't give
memory back to the OS.
Related issue: #4416

rtgui/CMakeLists.txt

@@ -286,6 +286,7 @@ target_link_libraries(rth rtengine
     ${ZLIB_LIBRARIES}
     ${LENSFUN_LIBRARIES}
     ${RSVG_LIBRARIES}
+    ${TCMALLOC_LIBRARIES}
     )
 
 target_link_libraries(rth-cli rtengine
@@ -307,6 +308,7 @@ target_link_libraries(rth-cli rtengine
     ${ZLIB_LIBRARIES}
     ${LENSFUN_LIBRARIES}
     ${RSVG_LIBRARIES}
+    ${TCMALLOC_LIBRARIES}
     )
 
 # Install executables