Although _LT_SYS_DYNAMIC_LINKER does support the [TAG] argument, almost all of the variables it does set are untagged variables.

While untagged variables are (expected to be) defined using the "C" language compiler, their values apply for each other language (=tag) as well.

As _LT_SYS_DYNAMIC_LINKER is run for multiple tags, these untagged variables are redefined again for no good reason. While this is not a problem by itself, it is a needless slowdown for the configure script.

However, a problem does arise when their values change with different language compilers, for example when using MSVC cl.exe as the C compiler (with Cygwin as the POSIX system):

./configure CC=cl.exe CXX='cl.exe /TP' results in libname_spec='ltdl' for the empty tag, which is incorporated into LTDLOPEN, causing libltdl/ltdl.c to refer lt_ltdl__LTX_preloaded_symbols.

But subsequent checks for F77 and FC run _LT_SYS_DYNAMIC_LINKER with $cc_basename != "cl.exe", causing libname_spec='libltdl', which finally ends up in the libtool script, causing lt_libltdl__LTX_preloaded_symbols to be generated.
Note "libltdl" != "ltdl" in the symbol name.

The error message seen here is:

   Creating library libltdl/.libs/libltdl.dll.lib and object libltdl/.libs/libltdl.dll.exp
libltdl_libltdl_la-ltdl.obj : error LNK2019: unresolved external symbol _lt_ltdl_LTX_preloaded_symbols referenced in function _lt_dlinit
libltdl/.libs/libltdl-7.dll : fatal error LNK1120: 1 unresolved externals
make[2]: *** [Makefile:1139: libltdl/libltdl.la] Error 2