meson: partial fix for building pzstd on MSVC

It uses non-portable compiler options unconditionally. Elsewhere, we
check the compiler ID and only add the right ones, globally. Do the same
here.

NDEBUG can actually be handled by a core option, so while we are moving
things around, do so.

Unfortunately, this doesn't fix things entirely. The remaining issue is
not Meson's issue though -- MSVC simply does not like this source code
and somehow chokes on innocent code with the inscrutable "syntax error"
and "illegal token".

build/meson/meson.build

@@ -104,8 +104,10 @@ use_lz4 = lz4_dep.found()
 
 add_project_arguments('-DXXH_NAMESPACE=ZSTD_', language: ['c'])
 
+pzstd_warning_flags = []
 if [compiler_gcc, compiler_clang].contains(cc_id)
   common_warning_flags = [ '-Wundef', '-Wshadow', '-Wcast-align', '-Wcast-qual' ]
+  pzstd_warning_flags = ['-Wno-shadow', '-Wno-deprecated-declarations']
   if cc_id == compiler_clang
     common_warning_flags += ['-Wconversion', '-Wno-sign-conversion', '-Wdocumentation']
   endif