[robbielyman]

This change removes -Dsystem_lua in favor of -fsys=system_lua. In order to find Lua, the user should pass --search-prefix {dir}, where Lua is installed in {dir}/lib. In the happy case that lua.h is present as {dir}/include/lua.h, no further action is needed. Otherwise (for example if lua.h is {dir}/include/lua/lua.h), the user should pass the path to the directory containing lua.h as an argument to -Dadditional_system_headers.

If -fsys=system_lua is present, the default for -Dshared is now true. Also, rather than panicking the configure stage, I added addFail steps.

[robbielyman]

Since this change is hard to test with CI, here is how I tested it: I have Lua 5.5 installed via homebrew at /opt/homebrew/lib/liblua5.5.dylib and, e.g. /opt/homebrew/include/lua/lua.h. I removed pkg-config (probably there are less extreme ways to test, but I was lazy) and compiled Ziglua as

zig build test -fsys=system_lua --search-prefix /opt/homebrew -Dadditional_system_headers=/opt/homebrew/include/lua

This change therefore does not remove additional_system_headers, since I figure a nonzero number of system installs will put lua.h in a funny spot. Interestingly, attempting to cover for that by changing #include "lua.h" to #include "lua/lua.h" didn't work.

[natecraddock]

Just tested this on Linux and it works well!

~/dev/sandbox/ziglua-test $ zig build -Doptimize=small
~/dev/sandbox/ziglua-test $ ldd zig-out/bin/test
	linux-vdso.so.1 (0x00007ffeb58a9000)
	libm.so.6 => /lib/x86_64-linux-gnu/libm.so.6 (0x00007e7c65ded000)
	libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007e7c65a00000)
	/lib64/ld-linux-x86-64.so.2 (0x00007e7c65eec000)
~/dev/sandbox/ziglua-test $ ll zig-out/bin/test
-rwxrwxr-x 1 nathan nathan 342K Oct 30 23:05 zig-out/bin/test*

~/dev/sandbox/ziglua-test $ zig build -fsys=lua -Doptimize=small
~/dev/sandbox/ziglua-test $ ldd zig-out/bin/test
	linux-vdso.so.1 (0x00007ffeaa4e6000)
	liblua5.4.so.0 => /lib/x86_64-linux-gnu/liblua5.4.so.0 (0x0000720787220000)
	libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x0000720787000000)
	/lib64/ld-linux-x86-64.so.2 (0x0000720787277000)
	libm.so.6 => /lib/x86_64-linux-gnu/libm.so.6 (0x0000720786f17000)
~/dev/sandbox/ziglua-test $ ll zig-out/bin/test
-rwxrwxr-x 1 nathan nathan 136K Oct 30 23:05 zig-out/bin/test*