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Curiously, GNU make has no feature for listing just the names of targets defined in a makefile. The -p option produces output that includes all targets, but buries them in a lot of other information.

Place the following rule in a makefile for GNU make to implement a target named list that simply lists all target names in alphabetical order - i.e.: invoke as make list:

.PHONY: list
list:
    @$(MAKE) -pRrq -f $(lastword $(MAKEFILE_LIST)) : 2>/dev/null | awk -v RS= -F: '/^# File/,/^# Finished Make data base/ {if ($$1 !~ "^[#.]") {print $$1}}' | sort | egrep -v -e '^[^[:alnum:]]' -e '^$@$$' | xargs

^{Note: On pasting this, make sure that the last line is indented by exactly 1 tab.}

Note that sorting the resulting list of targets is the best option, since not sorting doesn't produce a helpful ordering in that the order in which the targets appear in the makefile is not preserved.
Also, the sub-targets of a rule comprising multiple targets are invariably output separately and will therefore, due to sorting, usually not appear next to one another; e.g., a rule starting with a z: will not have targets a and z listed next to each other in the output, if there are additional targets.

Explanation of the rule:

• .PHONY: list
  • declares target list a phony target, i.e., one not referring to a file, which should therefore have its recipe invoked unconditionally
• $(MAKE) -prRn -f $(lastword $(MAKEFILE_LIST)) : 2>/dev/null
  • Invokes make again in order to print and parse the database derived from the makefile:
    • -p prints the database
    • -Rr suppresses inclusion of built-in rules and variables
    • -q only tests the up-to-date-status of a target (without remaking anything), but that by itself doesn't prevent execution of recipe commands in all cases; hence:
    • -f $(lastword $(MAKEFILE_LIST)) ensures that the same makefile is targeted as in the original invocation, regardless of whether it was targeted implicitly or explicitly with -f ....
      Caveat: this will break if your makefile contains include directives; to address this, define variable THIS_FILE := $(lastword $(MAKEFILE_LIST)) before any include directives and use -f $(THIS_FILE) instead.
    • : is a deliberately invalid target that is meant to ensure that no commands are executed; 2>/dev/null suppresses the resulting error message. Note: This relies on -p printing the database nonetheless, which is the case as of GNU make 3.82. Sadly, GNU make offers no direct option to just print the database.
• -v RS=
  • This is an awk idiom that breaks the input into blocks of contiguous non-empty lines.
• /^# File/,/^# Finished Make data base/
  • Matches the range of lines in the output that contains all targets (true as of GNU make 3.82) - by limiting parsing to this range, there is no need to deal with false positives from other output sections.
• if ($$1 !~ "^[#.]")
  • Selectively ignores blocks:
    • # ... ignores non-targets, whose blocks start with # Not a target:
    • . ... ignores special targets
  • All other blocks should each start with a line containing only the name of an explicitly defined target followed by :
• egrep -v -e '^[^[:alnum:]]' -e '^$@$$' removes unwanted targets from the output:
  • '^[^[:alnum:]]' ... excludes hidden targets, which - by convention - are targets that start neither with a letter nor a digit.
  • '^$@$$' ... excludes the list target itself
• xargs
  • Effectively converts the output lines to a single-line, space-separated list; omit this if you want each target name to appear on its own line.