[scons] adjustment for static build under CentOS

x-ref:
  "Trying to compile under CentOS 6.10"
  https://redmine.lighttpd.net/boards/2/topics/8572

src/SConscript

@@ -259,7 +259,8 @@ fullstatic_libs = GatherLibs(env, fullstaticlib)
 fullstatic_linkflags = [staticenv['LINKFLAGS'], '-static']
 if (('pthread' in fullstatic_libs) or ('pcre' in fullstatic_libs)) and (platform.system() == 'Linux'):
 	fullstatic_linkflags += ['-Wl,--whole-archive','-lpthread','-Wl,--no-whole-archive']
-	fullstatic_libs.remove('pthread')
+	if 'pthread' in fullstatic_libs:
+		fullstatic_libs.remove('pthread')
 if 'gcc_s' in fullstatic_libs:
 	fullstatic_linkflags += ['-static-libgcc']
 	fullstatic_libs.remove('gcc_s')