GUI fails to build without ICU

[rmottola]

ICU is not detected:

checking for icu-config... no
Could not find the icu-config script.
Please ensure that it is in your path.
See http://site.icu-project.org/ for help.
checking for icu-i18n > 49.0... no
configure: checking for libicu (icu-config disabled)...
no
configure: WARNING: The International Components for Unicode (ICU) development headers and libraries do not appear to be available on this system.

but it attempts anyway to link with it, failing:

 Linking library libgnustep-gui ...
/usr/bin/ld: cannot find -licui18n
/usr/bin/ld: cannot find -licuuc
/usr/bin/ld: cannot find -licudata
collect2: error: ld returned 1 exit status

[rmottola]

It builds if ICU is explicitely disabled with --disable-icu

I think this is because we force some kind of flags even if pkg-conf and icu-config are not present:

    LDFLAGS="$LDFLAGS $LIBS $ICU_LDFLAGS -licui18n -licuuc -licudata -lm"
    AC_LINK_IFELSE([AC_LANG_PROGRAM([[]], [[]])],[HAVE_ICU=1],[HAVE_ICU=0]);
    LDFLAGS="$saved_LDFLAGS";
    ICU_LIBS="-licui18n -licuuc -licudata -lm" 

I think this is a hack... but it makes the bug less "serious"