follow-up: build with Qt6 support on Linux with Qt 6.1

1d3178a quieted the g++ build, but made the warnings even
worse with a clang build.

The addition of the -Wno-sfinae-incomplete caused an unrecognized
option warning using recent clang.

Recent clang build with Qt6.1 also caused several warnings
during the processing of the Qt6.1 header files related to
c++26-extensions.

This adds (under Linux) -Wnoc++-26-extensions to the clang++
command line to quiet those warnings and restricts the
-Wno-sfinae-incomplete command line option to the g++ build.

sys/unix/hints/include/compiler.500

@@ -138,9 +138,6 @@ CCXXFLAGS+=-Wall -Wextra -Wno-missing-field-initializers \
         -Wreturn-type -Wunused -Wformat -Wswitch -Wshadow -Wwrite-strings
 CCXXFLAGS+=-pedantic
 CCXXFLAGS+=-Wmissing-declarations
-ifdef ADDITIONAL_NO_WARN
-CCXXFLAGS+=$(ADDITIONAL_NO_WARN)
-endif
 #detection of clang++ vs g++
 CXXISCLANG := $(shell echo `$(CXX) --version` | grep clang)
 ifeq "$(CXXISCLANG)" ""
@@ -179,6 +176,9 @@ else   # g++ version greater than or equal to 9? (no follows)
 CCXX=g++ -std=c++17
 endif  # g++ version greater than or equal to 9
 endif  # CPLUSPLUS_NEED20
+ifdef ADDITIONAL_NO_WARN_GPPCCX
+CCXXFLAGS+=$(ADDITIONAL_NO_WARN_GPPCCX)
+endif
 
 
 else   # g++ or clang++ ?
@@ -219,6 +219,9 @@ ifeq "$(CLANGPPGTEQ17)" "1"
 CCXX=clang++ -std=c++20
 endif  # clang++ greater than or equal to 17
 endif  # CPLUSPLUS_NEED20
+ifdef ADDITIONAL_NO_WARN_CLANGCCX
+CCXXFLAGS+=$(ADDITIONAL_NO_WARN_CLANGCCX)
+endif
 endif  # end of clang++-specific section
 CXX=$(CCXX)
 endif  # CPLUSPLUS_NEEDED

sys/unix/hints/linux.500

@@ -77,7 +77,8 @@ USE_NOSTATICFN = 1
 #QT6 prior to compiler include
 ifdef WANT_WIN_QT6
 CPLUSPLUS_NEED20=1
-ADDITIONAL_NO_WARN=-Wno-sfinae-incomplete
+ADDITIONAL_NO_WARN_GPPCCX=-Wno-sfinae-incomplete
+ADDITIONAL_NO_WARN_CLANGCCX=-Wno-c++26-extensions
 endif   # WANT_WIN_QT6
 
 # compiler.500 contains compiler detection and adjustments common