environ: fix link error on 64-bit Cygwin

[Bruno Haible]

On 64-bit Cygwin, a libunistring build fails like this:

$ /bin/sh ../libtool  --tag=CC --preserve-dup-deps  --mode=link 
x86_64-pc-cygwin-gcc  -g -O2  -L/usr/local/cygwin64/lib 
-Wl,--disable-auto-import -o test-environ.exe test-environ.o libtests.a 
../lib/libunistring.la libtests.a
libtool: link: x86_64-pc-cygwin-gcc -g -O2 -Wl,--disable-auto-import -o 
.libs/test-environ.exe test-environ.o  -L/usr/local/cygwin64/lib libtests.a 
../lib/.libs/libunistring.dll.a -liconv libtests.a -L/usr/local/cygwin64/lib
test-environ.o:test-environ.c:(.rdata$.refptr.environ[.refptr.environ]+0x0): 
undefined reference to `environ'
collect2: error: ld returned 1 exit status

This fixes it.


2018-01-31  Bruno Haible  <address@hidden>

        environ: Fix link error on 64-bit Cygwin.
        * lib/unistd.in.h (environ): On Cygwin, redeclare with the
        __declspec(dllimport) attribute.
        * doc/posix-functions/environ.texi: Mention the Cygwin problem.

diff --git a/doc/posix-functions/environ.texi b/doc/posix-functions/environ.texi
index 34ac25a..a6c0095 100644
--- a/doc/posix-functions/environ.texi
+++ b/doc/posix-functions/environ.texi
@@ -24,6 +24,9 @@ shared libraries on Mac OS X 10.5.  Here is a workaround: 
Instead, one can use
 #define environ (*_NSGetEnviron())
 @end smallexample
 This works at all versions of Mac OS X.
address@hidden
+On Cygwin in 64-bit mode, references to this variable cause a link error when
+the option @code{-Wl,--disable-auto-import} is in use.
 @end itemize
 
 Portability problems not fixed by Gnulib:
diff --git a/lib/unistd.in.h b/lib/unistd.in.h
index 6802e3b..4ef0ffa 100644
--- a/lib/unistd.in.h
+++ b/lib/unistd.in.h
@@ -400,6 +400,13 @@ _GL_WARN_ON_USE (dup3, "dup3 is unportable - "
 
 
 #if @GNULIB_ENVIRON@
+# if defined __CYGWIN__
+/* The 'environ' variable is defined in a DLL. Therefore its declaration needs
+   the '__declspec(dllimport)' attribute, but the system's <unistd.h> lacks it.
+   This leads to a link error on 64-bit Cygwin when the option
+   -Wl,--disable-auto-import is in use.  */
+_GL_EXTERN_C __declspec(dllimport) char **environ;
+# endif
 # if address@hidden@
 /* Set of environment variables and values.  An array of strings of the form
    "VARIABLE=VALUE", terminated with a NULL.  */