utils: Move thread-safe strerror replacement to a separate file

For some utils _GNU_SOURCE might be needed but that conflicts with the
signature of strerror_r(3).

src/libstrongswan/Makefile.am

@@ -34,7 +34,8 @@ selectors/traffic_selector.c threading/thread.c threading/thread_value.c \
 threading/mutex.c threading/semaphore.c threading/rwlock.c threading/spinlock.c \
 utils/utils.c utils/chunk.c utils/debug.c utils/enum.c utils/identification.c \
 utils/lexparser.c utils/optionsfrom.c utils/capabilities.c utils/backtrace.c \
-utils/settings.c utils/test.c
+utils/settings.c utils/test.c \
+utils/utils/strerror.c
 
 if USE_DEV_HEADERS
 strongswan_includedir = ${dev_headers}
@@ -82,7 +83,8 @@ utils/utils.h utils/chunk.h utils/debug.h utils/enum.h utils/identification.h \
 utils/lexparser.h utils/optionsfrom.h utils/capabilities.h utils/backtrace.h \
 utils/leak_detective.h utils/printf_hook/printf_hook.h \
 utils/printf_hook/printf_hook_vstr.h utils/printf_hook/printf_hook_builtin.h \
-utils/settings.h utils/test.h utils/integrity_checker.h
+utils/settings.h utils/test.h utils/integrity_checker.h \
+utils/utils/strerror.h
 endif
 
 library.lo :	$(top_builddir)/config.status