Make _add_system_libs reentrant

The method _add_system_libs of ModifyEnvBase is not reentrant, and fails
when called a second time. This happens, at least, in a CMake-based
recipe with use_system_libs = True. This patch fixes it assuming that,
when _old_env is not empty, system lib paths have already been added.
14 jobs for fix_add_system_libs in 193 minutes and 48 seconds (queued for 40 seconds)
Status Job ID Name Coverage
  Preparation
passed #387060
manifest

00:00:39

 
  Pre Build
passed #387062
cerbero deps android universal

00:52:40

passed #387061
cerbero deps fedora x86_64

00:37:35

passed #387066
gst-ios-12.2
cerbero deps ios universal

02:27:08

passed #387065
gst-macos-10.14
cerbero deps macos x86_64

00:48:21

passed #387063
cerbero deps windows x86

00:41:52

passed #387064
cerbero deps windows x86_64

00:51:16

 
  Build
passed #387068
cerbero android universal

00:14:35

passed #387069
cerbero cross win32

00:12:23

passed #387070
cerbero cross win64

00:05:49

passed #387067
cerbero fedora x86_64

00:08:08

passed #387072
gst-ios-12.2
cerbero ios universal

00:38:44

passed #387071
gst-macos-10.14
cerbero macos x86_64

00:08:24

 
  Integrate
passed #387073
cerbero android universal examples

00:07:16