Build system: fix detection of SLICOT for MATLAB on RHEL

The MATLAB_CFLAGS variable was not injected when testing for the presence of
libslicot64_pic. In particular, on GNU/Linux system, this means that the -fPIC
flag was not injected when doing the test. This was not a problem on Debian,
where GCC now enables -fPIC by default. But this would fail on RHEL, where
-fPIC is not enabled by default.

By the way:
— rename ac_save_* variables to my_save_*, to avoid conflicts with internal
  autoconf variables;
— do not inject “$(mkoctfile -p LFLAGS)” into LDFLAGS when testing for SLICOT
  under Octave; this is no longer necessary, since those flags are already
  injected at the beginning of mex/build/octave/configure.ac.

(cherry picked from commit 3d6f86c2)