Fix Lua header detection

Fix Lua header detection

checking lua.h usability... yes
checking lua.h presence... no
configure: WARNING: lua.h: accepted by the compiler, rejected by the
preprocessor!
configure: WARNING: lua.h: proceeding with the compiler's result
checking for lua.h... yes
checking lauxlib.h usability... yes
checking lauxlib.h presence... no
configure: WARNING: lauxlib.h: accepted by the compiler, rejected by the
preprocessor!
configure: WARNING: lauxlib.h: proceeding with the compiler's result
checking for lauxlib.h... yes
checking lualib.h usability... yes
checking lualib.h presence... no
configure: WARNING: lualib.h: accepted by the compiler, rejected by the
preprocessor!
configure: WARNING: lualib.h: proceeding with the compiler's result
checking for lualib.h... yes

AC_CHECK_HEADERS uses the preprocessor directly, which doesn't use
CFLAGS. Fixes detection of the lua-5.1 compat libs on Fedora Rawhide.