Fix MAKEFLAGS parsing with Make 4.4+

Since Make version 4.4, MAKEFLAGS also contains long options and
overridden variables on the command-line[1].

[1]: https://git.savannah.gnu.org/cgit/make.git/tree/NEWS?h=4.4#n67

This means that parsing by filtering out '--%' doesn't work reliably
anymore, since it doesn't remove overrides:

Running 'make VAR=quacks' gives 'MAKEFLAGS=" -- VAR=quacks"', and
'checkflags' would understand that flags -q, -s, ... were set.

This would get transmitted below into 'QUIET_FLAGS' and
'QUESTION_FLAG', then passed to the 'genVersionHeader.pl' as '-i' and
'-q'.

The result would be that 'genVersionHeader.pl' would never create the
version header (only check for its up-to-date status), leading to
confusing errors:

  ../misc/epicsRelease.c:25:32: error: expected ')' before 'EPICS_VCS_VERSION'
     25 |     printf ( "## %s\n", "Rev. " EPICS_VCS_VERSION );
        |            ~                   ^~~~~~~~~~~~~~~~~~

The NEWS file[1] recommends using 'firstword', but unfortunately this is
not compatible with GNUMake < 3.82.

configure/CONFIG_COMMON

@@ -83,8 +83,9 @@ IOCS_APPL_TOP               = $(INSTALL_ABSOLUTE)
 
 #-------------------------------------------------------
 # How to portably check the flags to make
+makeflags := $(firstword $(filter-out -,$(filter-out --%,$(MAKEFLAGS))))
 define checkflags
-  make-$1 = $(findstring $1,$(filter-out --%,$(MAKEFLAGS)))
+  make-$1 := $(findstring $1,$(makeflags))
 endef
 # This is extensible to most single letter flags:
 $(foreach flag,s q, $(eval $(call checkflags,$(flag))))