autogen: Fix MOC_PREDEFS_CMD if CXX env var contains multiple values

Since commit 429a4527 we inject the C++ standard argument into
MOC_PREDEFS_CMD.  That failed if the CXX environment variable was set to
a value like "ccache c++", because the argument was inserted between
"ccache" and "c++".

Fix this by taking CMAKE_CXX_COMPILER_ARG1 into account that contains
the compiler arguments in CXX.

Fixes: #27445