How-to 'make install' a static library from debian/rules to be used on the main program compile as a dependency?

The idea is to nest/merge a (static) library on the launchpad recipe i'm trying to build (launchpad uses pbuilder) and then build it from source and 'make install' so that the dependency that the main program needs is installed in the build env even though it it still not on the ubuntu repositories.

Doing it like this:

    #!/usr/bin/make -f
    export PREFIX=/usr
    export CFLAGS= -O3 -fomit-frame-pointer -flto -fwhole-program
    export CXXFLAGS= -O3 -fomit-frame-pointer -flto -fwhole-program

    %:
        dh $@
    override_dh_auto_configure:
        cd src/munt;cmake -DCMAKE_CXX_FLAGS="-O3 -fomit-frame-pointer -flto" mt32emu;make;make install
#...compile of the program that depends on mt32emu...

but it give this error:
Install the project...
-- Install configuration: ""
-- Installing: /usr/local/lib/libmt32emu.a
CMake Error at cmake_install.cmake:36 (FILE):
  file INSTALL cannot copy file
  "/tmp/buildd/dosbox-0.74+20121225/src/munt/libmt32emu.a" to
  "/usr/local/lib/libmt32emu.a".

make[2]: *** [install] Error 1
make[2]: Leaving directory `/tmp/buildd/dosbox-0.74+20121225/src/munt'
make[1]: *** [override_dh_auto_configure] Error 2
make[1]: Leaving directory `/tmp/buildd/dosbox-0.74+20121225'
make: *** [build] Error 2
dpkg-buildpackage: error: debian/rules build gave error exit status 2
E: Failed autobuilding of package
I: unmounting /var/cache/pbuilder/ccache filesystem
I: unmounting dev/pts filesystem
I: unmounting proc filesystem
I: cleaning the build env
I: removing directory /var/cache/pbuilder/build//2751 and its subdirectories

If it try to use 'sudo make install' (and add sudo to the build-deps in debian/control), on the local tests i did, it asks for the 'pbuilder' password, which i don't know, and which will probably fail on launchpad no?