scons: Don't explicitly add tcmalloc_minimal to LIBS

SCons automatically adds a library to LIBS if conf.CheckLib succeeds,
so there is no need to explicitly add the library.

SConstruct

@@ -914,10 +914,7 @@ have_posix_clock = \
     conf.CheckLibWithHeader('rt', 'time.h', 'C',
                             'clock_nanosleep(0,0,NULL,NULL);')
 
-if conf.CheckLib('tcmalloc_minimal'):
-    have_tcmalloc = True
-else:
-    have_tcmalloc = False
+if not conf.CheckLib('tcmalloc_minimal'):
     print termcap.Yellow + termcap.Bold + \
           "You can get a 12% performance improvement by installing tcmalloc "\
           "(libgoogle-perftools-dev package on Ubuntu or RedHat)." + \
@@ -1199,9 +1196,6 @@ for variant_path in variant_paths:
     if env['USE_SSE2']:
         env.Append(CCFLAGS=['-msse2'])
 
-    if have_tcmalloc:
-        env.Append(LIBS=['tcmalloc_minimal'])
-
     # The src/SConscript file sets up the build rules in 'env' according
     # to the configured variables.  It returns a list of environments,
     # one for each variant build (debug, opt, etc.)