FreeBSD build possible, modifications needed

cflags, lflags, and defines need to be FreeBSD specific in this case:

``` diff
diff --git a/project.janet b/project.janet
index 96e9e81..d954222 100644
--- a/project.janet
+++ b/project.janet
@@ -16,6 +16,7 @@
:windows '["user32.lib" "gdi32.lib" "winmm.lib" "shell32.lib"]
:macos '["-lpthread" "-framework" "Cocoa" "-framework" "CoreVideo" "-framework" "IOKit" "-framework" "OpenGL"]
:linux '["-lpthread" "-lX11"]
+ :freebsd '["-L/usr/local/lib" "-lpthread" "-lX11"]
#default
'["-lpthread"]))

@@ -24,7 +25,7 @@

:cflags [;default-cflags ;cflags]

- :defines {"PLATFORM_DESKTOP" true "_POSIX_C_SOURCE" "200809L"}
+ :defines {"PLATFORM_DESKTOP" true } # "_POSIX_C_SOURCE" "200809L"}

:source ["src/main.c"

@@ -47,6 +48,8 @@
"src/shapes.h"
"src/3d.h"]

+ :cflags [;default-cflags ;'["-I/usr/local/include"]]
+
:lflags [;default-lflags ;lflags])

# `jpm run repl` to run a repl with access to jaylib
```

That illustrates the changes necessary, however, is not fit for actual use (would probably break other platforms) so is mostly provided as documentation.

The other ordeal might be that the cflags, lflags, should be defaults under FreeBSD in jpm but am not sure if that is true or not. Depends on what those are under other platforms. The `_POSIX_C_SOURCE` define breaks `soundcard.h` on some platforms supposedly (currently on FreeBSD to our knowledge).

Start in project.janet and inspect how jpm supplies platform-specific cflags, lflags, and defines for FreeBSD and other platforms. Compare the proposed FreeBSD values, including _POSIX_C_SOURCE and /usr/local paths, against existing platform settings. Done means FreeBSD builds successfully without breaking other supported platforms.

Written by the indexing model from the issue text.