Per the GNU make manual, function call syntax is either
  $(function arguments)  # parenthesis
or
  ${function arguments}  # braces

Using braces in most cases works, however, using braces inside an `ifeq` or `ifneq` condition sometimes fails with:

* unterminated call to function 'join': missing '}'.  Stop.

It fails if the function is the 1st argument of the `ifeq`, and the function has multiple arguments (e.g., findstring). It works if the function is the 2nd argument of `ifeq`, or the function has only one argument (e.g., sort). Illustrative examples (more in attached Makefile):

```
#-------------------------------------------------------------------------------
ifeq ($(and a),)
endif

# ok
ifeq (${and a},)
endif

# ok
ifneq (,${and a})
endif

# ok
ifneq (${and a},)
endif

# ok
ifneq (,${and a})
endif

#-------------------------------------------------------------------------------
ifeq ($(and a, b),)
endif

# Makefile:469: * unterminated call to function 'and': missing '}'.  Stop.
## ifeq (${and a, b},)
## endif

# ok
ifneq (,${and a, b})
endif

# Makefile:477: * unterminated call to function 'and': missing '}'.  Stop.
## ifneq (${and a, b},)
## endif

# ok
ifneq (,${and a, b})
endif
```