m4/as-linux.m4: Add extra quoting in AS_CHECK_REDHAT_PRE_FC2 macro

m4/as-linux.m4: Add extra quoting in AS_CHECK_REDHAT_PRE_FC2 macro

Avoid the following warning:

$ ./autogen.sh
configure.ac:42: warning: AC_LANG_CONFTEST: no AC_LANG_SOURCE call
detected in body
../../lib/autoconf/lang.m4:193: AC_LANG_CONFTEST is expanded from...
../../lib/autoconf/general.m4:2590: _AC_COMPILE_IFELSE is expanded
from...
../../lib/autoconf/general.m4:2606: AC_COMPILE_IFELSE is expanded
from...
m4/as-linux.m4:435: AS_CHECK_REDHAT_PRE_FC2 is expanded from...
m4/as-linux.m4:69: AS_LINUX is expanded from...
configure.ac:42: the top level

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>