Fix check for libstdc++

...now that we have HAVE_LIBSTDCPP since
7158acde77 "Consolidate -D_GLIBCXX_DEBUG for
--enable-dbgutil builds against libstdc++".  HAVE_LIBCXX was only ever defined
in config_host/config_libcxx.h (since 90d2155912
"Work around latest Boost 1.65.1 still using deprecated std::auto_ptr"), never
as a makefile variable, so this check (introduced in
b1141fa610 "Cut down on -pthread/-lpthread
proliferation") always succeeded, whether libstdc++ or not.

Change-Id: If71615b0ae2c6d3765eb538857f36bb1d23a667a
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/124683
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>

solenv/gbuild/platform/unxgcc.mk

@@ -42,10 +42,10 @@ gb_CFLAGS := \
 	-Wdeclaration-after-statement \
 	-Wshadow \
 
-# At least libstdc++ (which is approximated here with !HAVE_LIBCXX) needs -pthread when including
-# various C++ headers like <thread>, see <https://gcc.gnu.org/onlinedocs/gcc-8.3.0/libstdc++/manual/
-# manual/using.html#manual.intro.using.flags>:
-ifeq ($(HAVE_LIBCXX),)
+# At least libstdc++ needs -pthread when including various C++ headers like <thread>, see
+# <https://gcc.gnu.org/onlinedocs/gcc-8.3.0/libstdc++/manual/manual/using.html
+# #manual.intro.using.flags>:
+ifneq ($(HAVE_LIBSTDCPP),)
 gb_CXX_LINKFLAGS := -pthread
 endif