Fix install-strip target

Fix install-strip target

The libtool patch broke install-strip of gdb:

/bin/sh ../../gdb/../mkinstalldirs /src/gdb/inst/share/gdb/python/gdb
transformed_name=`t='s,y,y,'; \
                  echo gdb | sed -e "$t"` ; \
        if test "x$transformed_name" = x; then \
          transformed_name=gdb ; \
        else \
          true ; \
        fi ; \
        /bin/sh ../../gdb/../mkinstalldirs /src/gdb/inst/bin ; \
        /bin/sh ./libtool --mode=install STRIPPROG='strip' /bin/sh /src/gdb/gdb.git/install-sh -c -s \
                gdb \
                /src/gdb/inst/bin/$transformed_name ; \
        /bin/sh ../../gdb/../mkinstalldirs /src/gdb/inst/include/gdb ; \
        /usr/bin/install -c -m 644 jit-reader.h /src/gdb/inst/include/gdb/jit-reader.h
libtool: install: `/src/gdb/inst/bin/gdb' is not a directory
libtool: install: Try `libtool --help --mode=install' for more information.

Since INSTALL_PROGRAM_ENV is no longer at the beginning of the command, the
gdb executable is not installed with install-strip.