Cygwin: uname: fix building in non-git source dir

commit 97eb64b909 broke building outside of a git dir, because
the git describe command would fail.  Fix this by checking if
we're in a git tree at all and just generate an empty string
as version string.  Use this in uname_x to generate a fallback
version.

Fixes: 97eb64b909 ("Cygwin: uname: generate default release
string from git as well"
Signed-off-by: Corinna Vinschen <corinna@vinschen.de>

winsup/cygwin/Makefile.am

@@ -421,7 +421,10 @@ src_files := $(foreach dir,$(dirs),$(find_src_files))
 
 uname_version.c: .FORCE
 	$(AM_V_GEN)cd $(srcdir) && \
-	echo "const char *uname_dev_version = \"$$(git describe --dirty | sed -e 's/cygwin-//')\";" > $(abs_builddir)/uname_version.c
+	echo "const char *uname_dev_version = \
+	\"$$(git rev-parse --git-dir >/dev/null 2>&1 && \
+	     git describe --dirty | sed -e 's/cygwin-//')\";" \
+	     > $(abs_builddir)/uname_version.c
 .FORCE:
 
 # mkvers.sh creates version.cc in the first place, winver.o always

winsup/cygwin/uname.cc

@@ -60,8 +60,16 @@ uname_x (struct utsname *name)
 		__XSTRING (CYGPORT_RELEASE_INFO), name->machine);
 #else
       extern const char *uname_dev_version;
+      if (uname_dev_version && uname_dev_version[0])
 	snprintf (name->release, _UTSNAME_LENGTH, "%s.%s",
 		  uname_dev_version, name->machine);
+      else
+	__small_sprintf (name->release, "%d.%d.%d-api-%d.%s",
+			 cygwin_version.dll_major / 1000,
+			 cygwin_version.dll_major % 1000,
+			 cygwin_version.dll_minor,
+			 cygwin_version.api_minor,
+			 name->machine);
 #endif
 #pragma GCC diagnostic pop
       /* version */