m4/ax_detect_clang.m4: modify LIBS not LDFLAGS in check for clangEdit

m4/ax_detect_clang.m4: modify LIBS not LDFLAGS in check for clangEdit

CLANG_LIBS was originally added to LDFLAGS in pet-0.11.3-7-g12e72ff
(improve detection of clangEdit, Thu Jul 9 10:16:34 2020 +0200),
even though LIBS is arguably the more appropriate variable to modify,
perhaps because back then only LDFLAGS was being saved.
Since pet-0.11.6-16-g01606cc (when linking against libclang-cpp,
check linking of LLVM libs, Fri Mar 31 22:40:30 2023 +0200),
LIBS is also being saved and modified, so it can also be updated
to add CLANG_LIBS.

Note that there is no need to add CLANG_LDFLAGS to LDFLAGS (again)
since LDFLAGS is already being updated this way before the check
for libclang-cpp.

Signed-off-by: Sven Verdoolaege <sven.verdoolaege@gmail.com>