configure.py: prevent deduplication of seastar compile options

In its infinite wisdom, CMake deduplicates the options passed
to `target_compile_options`, making it impossible to pass options which require
duplication, such as -mllvm.
Passing e.g.
`-mllvm;-pgso=false;-mllvm;-inline-threshold=2500` invokes the compiler
`-mllvm -pgso=false -inline-threshold=2500`, breaking the options.

As a workaround, CMake added the `SHELL:` syntax, which makes it possible to
pass the list of options not as a CMake list, but as a shell-quoted string.
Let's use it, so we can pass multiple -mllvm options.

configure.py

@@ -1593,7 +1593,7 @@ def configure_seastar(build_dir, mode, mode_config):
         '-DCMAKE_C_COMPILER={}'.format(args.cc),
         '-DCMAKE_CXX_COMPILER={}'.format(args.cxx),
         '-DCMAKE_EXPORT_NO_PACKAGE_REGISTRY=ON',
-        '-DSeastar_CXX_FLAGS={}'.format((mode_config['lib_cflags']).replace(' ', ';')),
+        '-DSeastar_CXX_FLAGS=SHELL:{}'.format(mode_config['lib_cflags']),
         '-DSeastar_LD_FLAGS={}'.format(semicolon_separated(mode_config['lib_ldflags'], mode_config['cxx_ld_flags'])),
         '-DSeastar_CXX_DIALECT=gnu++20',
         '-DSeastar_API_LEVEL=6',