Make the "all" Makefile target more robust

This is a minor improvement to the robustness of the command for
"make all", in the face of plausible future target names.

- Use [[:alpha:]] instead of [a-z] because, in POSIX BRE and ERE,
  whether [a-z] includes capital letters depends on the current
  collation order. (The goal here is greater consistency across
  systems, and for this it would also be okay to use [[:lower:]].)

- Pass -x to the command that filters out the all target itself,
  so that the entire name must be "all", because a future target
  may contain the substring "all" as part of a larger word.

Author: EliahKagan

Makefile

@@ -1,7 +1,7 @@
 .PHONY: all clean release force_release
 
 all:
-	@grep -Ee '^[a-z].*:' Makefile | cut -d: -f1 | grep -vF all
+	@grep -E '^[[:alpha:]].*:' Makefile | cut -d: -f1 | grep -vxF all
 
 clean:
 	rm -rf build/ dist/ .eggs/ .tox/