external: Use gb_LTOFLAGS only in LDFLAGS to fix building.

liborcus was not building for me with -flto in CFLAGS, I would have to
fix ar somehow.
-flto in LDFLAGS is just to fix the build if the external library does use
another library built by us with -flto: does happen for liborcus and python3.
It's not like we would use -flto for external libraries consistently anyway,
the only exception is icu: no idea why we build with -flto there.

Change-Id: Ia54d619674b8999ce5e4b920ba77b1587c9cf48d

external/liborcus/ExternalProject_liborcus.mk

@@ -68,7 +68,7 @@ liborcus_CPPFLAGS+=-D_GLIBCXX_DEBUG
 endif
 endif
 
-liborcus_CXXFLAGS=$(CXXFLAGS) $(gb_LTOFLAGS)
+liborcus_CXXFLAGS=$(CXXFLAGS)
 liborcus_LDFLAGS=$(LDFLAGS) $(gb_LTOFLAGS)
 ifeq ($(COM),MSC)
 liborcus_CXXFLAGS+=$(BOOST_CXXFLAGS)

external/python3/ExternalProject_python3.mk

@@ -51,7 +51,7 @@ else
 # create a symlink "LO_lib" because the .so are in a directory with platform
 # specific name like build/lib.linux-x86_64-3.3
 
-python3_cflags = $(ZLIB_CFLAGS) $(gb_LTOFLAGS)
+python3_cflags = $(ZLIB_CFLAGS)
 ifneq (,$(ENABLE_VALGRIND))
     python3_cflags += $(VALGRIND_CFLAGS)
 endif