Trac #29532: cygwin: Fix spkg-configure.m4 for iconv

Follow-up from #27823 (spkg-configure.m4 for iconv), comment 46:

  It seems that, at least on Cygwin, AM_ICONV will pass, somehow, even
if iconv.h is installed, thus making the libicon-devel package on Cygwin
a requirement, at least when it comes to building R (which otherwise
fails at #include <iconv.h>).

  For now I'll just update the Cygwin build instructions to require
libiconv-devel as a prerequisite which is preferable anyways. But maybe
this still needs to be checked for somehow...

Indeed this needs fixing:
{{{
configure:21635: g++ -std=gnu++11 -o conftest.exe -g -O2    conftest.cpp
-lglpk -lreadline -lcurl -lbz2 -lflint -lmpfr -lgmp -lm  -lntl -liconv
>&5
conftest.cpp:68:10: fatal error: iconv.h: No such file or directory
   68 | #include <iconv.h>
      |          ^~~~~~~~~
compilation terminated.
...

configure:21645: result: no, consider installing GNU libiconv
## ------------------------------------------------------ ##
## Checking whether SageMath should install SPKG iconv... ##
## ------------------------------------------------------ ##
configure:21940: will use system package and not install SPKG iconv
## ---------------------------------------------------- ##

}}}

URL: https://trac.sagemath.org/29532
Reported by: mkoeppe
Ticket author(s): Matthias Koeppe
Reviewer(s): Dima Pasechnik

Author: Release Manager

build/pkgs/iconv/spkg-configure.m4

@@ -1,6 +1,6 @@
 SAGE_SPKG_CONFIGURE([iconv], [
     AM_ICONV
-    if test x$am_cv_func_iconv != xyes; then
+    if test x"$am_cv_func_iconv" != xyes; then
        sage_spkg_install_iconv=yes
     fi
 ])