Deal with Makefile conditional assignment operators.

Ordinarily variable assignments in Makefiles are done simply with an
unconditional assignment: the equals sign.  There are variants of this:

FOO := $(BAR)	Avoid recursively expand variables.
FOO ?= bar	Assign only if $(FOO) is not already set.
FOO += bar	Add more to a variable.
FOO != bar	Execute a shell script and assign result to $(FOO).

See also
https://www.gnu.org/software/make/manual/html_node/Conditional-Example.html
https://www.gnu.org/software/make/manual/html_node/Flavors.html

Library/Homebrew/extend/string.rb

@@ -41,7 +41,7 @@ module StringInreplaceExtension
   # Looks for Makefile style variable definitions and replaces the
   # value with "new_value", or removes the definition entirely.
   def change_make_var!(flag, new_value)
-    return if gsub!(/^#{Regexp.escape(flag)}[ \t]*=[ \t]*(.*)$/, "#{flag}=#{new_value}", false)
+    return if gsub!(/^#{Regexp.escape(flag)}[ \t]*[\\?\+\:\!]?=[ \t]*(.*)$/, "#{flag}=#{new_value}", false)
 
     errors << "expected to change #{flag.inspect} to #{new_value.inspect}"
   end
@@ -50,12 +50,14 @@ module StringInreplaceExtension
   def remove_make_var!(flags)
     Array(flags).each do |flag|
       # Also remove trailing \n, if present.
-      errors << "expected to remove #{flag.inspect}" unless gsub!(/^#{Regexp.escape(flag)}[ \t]*=.*$\n?/, "", false)
+      if gsub!(/^#{Regexp.escape(flag)}[ \t]*[\\?\+\:\!]?=.*$\n?/, "", false)
+        errors << "expected to remove #{flag.inspect}"
+      end
     end
   end
 
   # Finds the specified variable
   def get_make_var(flag)
-    self[/^#{Regexp.escape(flag)}[ \t]*=[ \t]*(.*)$/, 1]
+    self[/^#{Regexp.escape(flag)}[ \t]*[\\?\+\:\!]?=[ \t]*(.*)$/, 1]
   end
 end