When creating a shared library, but not when compiling or creating a program, libtool drops some flags from the command line provided by the user. This is done because flags unknown to libtool may interfere with library creation or require additional support from libtool, and because omitting flags is usually the conservative choice for a successful build.
If you encounter flags that you think are useful to pass, as a
work-around you can prepend flags with
to allow them to be passed through to the compiler driver
(see Link mode). Another possibility is to add flags already
to the compiler command at configure run time:
./configure CC='gcc -m64'
If you think libtool should let some flag through by default,
here's how you can test such an inclusion: grab the Libtool development
tree, edit the ltmain.m4sh file in the libltdl/config
subdirectory to pass through the flag (search for ‘Flags to be
passed through’), re-bootstrap and build with the flags in question
CXXFLAGS, etc. on the
configure command line as appropriate. Run the testsuite
as described in the README file and report results to
the Libtool bug reporting address firstname.lastname@example.org.