Ignore 'compile' script

The previous commit (ca03d49) added 'AM_PROG_CC_C_O' to 'Makefile.am'.
This causes Automake to create an extra script, 'compile', a ‘Wrapper
for compilers which do not understand '-c -o'’.  As an an autogenerated
file, this needs to be ignored.
Link against librt if necessary

For a while, we were very confused why ppamltracer would work happily on
some Debian systems but produce dynamic linker errors on other, similar
ones.  It turns out the difference was glibc versions – if you have
glibc <2.17, you need to specify '-lrt' for one of the functions
ppamltracer uses.  This commit adds an Autoconf macro that adds that
flag if necessary; it should improve portability in general as well.