I've found ${_+_} in the FreeBSD Makefile.
What is the meaning of ${_+_} in BSD Makefiles?
${_+_} refers to a variable. It is defined in share/mk/sys.mk which gets read by the make process. So user Zack pointed to the right direction in his comment; this variable expands to a + sign in some cases, depending on the flags given to make:
.if !empty(.MAKEFLAGS:M-n) && ${.MAKEFLAGS:M-n} == "-n"
_+_ ?=
.else
_+_ ?= +
.endif
The rationale for this can be found in the comment for this commit and this one, where the symbol was introduced:
Make make recurse into sub-directories and sub-makes when given
two -n flags. If only one -n flag is given the old behaviour
is retained (POLA).