Makefile: fix 'lesson-fixme' target for Windows (datacarpentry#486)

Git for Windows doesn't provide fgrep, which is a shortcut
to call `grep -F` on Mac and Linux. Instead, we have to use
full arguments.

Makefile

@@ -146,7 +146,7 @@ lesson-files :
 
 ## * lesson-fixme     : show FIXME markers embedded in source files
 lesson-fixme :
-	@fgrep -i -n FIXME ${MARKDOWN_SRC} || true
+	@grep --fixed-strings --word-regexp --line-number --no-messages FIXME ${MARKDOWN_SRC} || true
 
 ##
 ## IV. Auxililary (plumbing) commands