configure: overwrite "$TMP.c" when checking features

Just in case it already exists.
fix pppox compile problems on older kernels
fix caif compile problems on older kernels
fix if_alg compile problems on older kernels
fix rds compile problems on older kernels
add stub for LLC_OPT_PKTINFO

The definition of LLC_OPT_PKTINFO is absent on some older stable enterprise
kernels like rhel6. The patch adds stub for it to fix compile problems there.
remove signal.h check

this was there purely as an example
Add PF_CAIF and PF_ALG for compatibility with glibc 2.12 and older.

This patch fixes these build errors with glibc-2.12 or older.

  CC    sockaddr.o
sockaddr.c: In function ‘gen_caif’:
sockaddr.c:585: error: ‘PF_CAIF’ undeclared (first use in this function)
sockaddr.c:585: error: (Each undeclared identifier is reported only once
sockaddr.c:585: error: for each function it appears in.)
sockaddr.c: In function ‘gen_alg’:
sockaddr.c:609: error: ‘PF_ALG’ undeclared (first use in this function)
sockaddr.c: In function ‘generate_sockaddr’:
sockaddr.c:788: error: ‘PF_CAIF’ undeclared (first use in this function)
sockaddr.c:792: error: ‘PF_ALG’ undeclared (first use in this function)

Signed-off-by: Vinson Lee <vlee@twitter.com>
compat.h: add RDS constants

Signed-off-by: Jiri Slaby <jslaby@suse.cz>