libc/glibc: fix installing syscall.h when installing start-files.

Reported-by: Rafael C <groups.r2@gmail.com>
Reported-by: Jérôme BARDON <bardon.pro@gmail.com>
Reported-by: Daniel Price <daniel.price@gmail.com>
[yann.morin.1998@free.fr: use a conditional approach, also suggested by Daniel]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>

 CT_EXPERIMENTAL=y

 CT_LOCAL_TARBALLS_DIR="${HOME}/src"

 CT_SAVE_TARBALLS=y

 CT_CONFIG_SHELL_SH=y

 CT_LOG_EXTRA=y

 CT_ARCH_avr32=y

 CT_BINUTILS_V_2_18a=y

 CT_CC_V_4_2_2=y

 CT_LIBC_NEWLIB_V_1_17_0=y