float: Fix compilation error when gnulib's float.h exists twice

[Bruno Haible]

Santiago Vila reported a build of gettext-0.21, which failed due to gnulib's
generated float.h:

/bin/bash ../libtool  --tag=CC   --mode=compile gcc 
-DLOCALEDIR=\"/usr/share/locale\" -DBISON_LOCALEDIR=\"/usr/share/locale\" 
-DLOCALE_ALIAS_PATH=\"/usr/share/locale\" -DUSEJAVA=0 
-DGETTEXTJAR=\"/usr/share/gettext/gettext.jar\" 
-DLIBDIR=\"/usr/lib/powerpc64le-linux-gnu\" 
-DGETTEXTDATADIR=\"/usr/share/gettext\" 
-DPROJECTSDIR=\"/usr/share/gettext/projects\" -DEXEEXT=\"\" -DHAVE_CONFIG_H -I. 
-I..  -I. -I. -I.. -I.. -I../libgrep -I../gnulib-lib -I../gnulib-lib -I../intl 
-I../../gettext-runtime/intl -I../libgettextpo -I../libgettextpo 
-I///usr/include/libxml2  -Wdate-time -D_FORTIFY_SOURCE=2  -g -O2 
-fdebug-prefix-map=/<<PKGBUILDDIR>>=. -fstack-protector-strong -Wformat 
-Werror=format-security -c -o libgettextsrc_la-format-kde-kuit.lo `test -f 
'format-kde-kuit.c' || echo './'`format-kde-kuit.c
libtool: compile:  gcc -DLOCALEDIR=\"/usr/share/locale\" 
-DBISON_LOCALEDIR=\"/usr/share/locale\" 
-DLOCALE_ALIAS_PATH=\"/usr/share/locale\" -DUSEJAVA=0 
-DGETTEXTJAR=\"/usr/share/gettext/gettext.jar\" 
-DLIBDIR=\"/usr/lib/powerpc64le-linux-gnu\" 
-DGETTEXTDATADIR=\"/usr/share/gettext\" 
-DPROJECTSDIR=\"/usr/share/gettext/projects\" -DEXEEXT=\"\" -DHAVE_CONFIG_H -I. 
-I.. -I. -I. -I.. -I.. -I../libgrep -I../gnulib-lib -I../gnulib-lib -I../intl 
-I../../gettext-runtime/intl -I../libgettextpo -I../libgettextpo 
-I///usr/include/libxml2 -Wdate-time -D_FORTIFY_SOURCE=2 -g -O2 
-fdebug-prefix-map=/<<PKGBUILDDIR>>=. -fstack-protector-strong -Wformat 
-Werror=format-security -c format-kde-kuit.c  -fPIC -DPIC -o 
.libs/libgettextsrc_la-format-kde-kuit.o
In file included from /usr/include/unicode/utypes.h:41,
                 from /usr/include/unicode/ucnv_err.h:88,
                 from /usr/include/unicode/ucnv.h:52,
                 from ///usr/include/libxml2/libxml/encoding.h:31,
                 from ///usr/include/libxml2/libxml/parser.h:810,
                 from format-kde-kuit.c:43:
../gnulib-lib/float.h:150:7: error: redefinition of ‘union gl_long_double_union’
  150 | union gl_long_double_union
      |       ^~~~~~~~~~~~~~~~~~~~
In file included from ../gnulib-lib/float.h:27,
                 from /usr/include/unicode/utypes.h:41,
                 from /usr/include/unicode/ucnv_err.h:88,
                 from /usr/include/unicode/ucnv.h:52,
                 from ///usr/include/libxml2/libxml/encoding.h:31,
                 from ///usr/include/libxml2/libxml/parser.h:810,
                 from format-kde-kuit.c:43:
../libgettextpo/float.h:150:7: note: originally defined here
  150 | union gl_long_double_union
      |       ^~~~~~~~~~~~~~~~~~~~

This patch should fix it.


2020-12-20  Bruno Haible  <bruno@clisp.org>

        float: Fix compilation error when gnulib's float.h exists twice.
        Reported by Santiago Vila <sanvila@unex.es>.
        * lib/float.in.h (GNULIB_defined_long_double_union): New macro.

diff --git a/lib/float.in.h b/lib/float.in.h
index 8e4d47d..7988cc7 100644
--- a/lib/float.in.h
+++ b/lib/float.in.h
@@ -93,11 +93,14 @@
      extern const long double LDBL_MAX;
 
    Unfortunately, this is not a constant expression.  */
+# if !GNULIB_defined_long_double_union
 union gl_long_double_union
   {
     struct { unsigned int lo; unsigned int hi; unsigned int exponent; } xd;
     long double ld;
   };
+#  define GNULIB_defined_long_double_union 1
+# endif
 extern const union gl_long_double_union gl_LDBL_MAX;
 # define LDBL_MAX (gl_LDBL_MAX.ld)
 /* Minimum e such that 10^e is in the range of normalized numbers.  */