BUILD: makefile: fix build error with GNU make 4.2.1 and /bin/dash

BUILD: makefile: fix build error with GNU make 4.2.1 and /bin/dash

The latest fix in the Makefile in commit 9993688954 ("BUILD: makefile/lua:
use the system's default library before all other variants") broke the
build on a machine with GNU make 4.2.1 and /bin/dash:

  Makefile:690: *** unterminated call to function 'shell': missing ')'.  Stop.

It's caused by the '#' in '#include'. Protecting it with a backslash
fixes the make issue but moves it to the shell where it's echoed in the
output. Printf '\043' works but not sure if it's everywhere yet. At this
point better just revert that tiny part which was made to refine the
presence check for lua.h by checking that it contains valid C code. If
the commit above is backported, this one will have to be as well.