Review parser Makefile (#2765)

* parser: Fail gracefully if a nonexistent file is passed on the command line

Before, if a nonexistent file was passed to LoadFileText(), it would
return NULL, and the parser would happily dereference it.

* parser: Refactor Makefile and update the path to easings.h (now reasings.h)

Before, the `make all` target would simply segfault, see 0a679d79
Now, if a file in the `make all` target doesn't exist, make will write
an error.

Individual API files can be generated likeso, provided the header file
the target depends on exists:

FORMAT=JSON EXTENSION=json make raygui_api.json

In order for the `make all` target to succeed, raygui.h, physac.h and
rmem.h need to be added to the correct directory.

parser/raylib_parser.c

@@ -207,6 +207,12 @@ int main(int argc, char* argv[])
     int length = 0;
     char *buffer = LoadFileText(inFileName, &length);
 
+    if (buffer == NULL)
+    {
+	    printf("Could not read input file: %s\n", inFileName);
+	    return 1;
+    }
+
     // Preprocess buffer to get separate lines
     // NOTE: GetTextLines() also removes leading spaces/tabs
     int linesCount = 0;