Either -O2 is broken in Apple Clang, or some of our code is broken

Already with Xcode 7.3.1 some unit tests crash when the code is built
with full optimisation. Xcode 8 did not help. So use -O1. Yeah, this
sucks. Don't have time to investigate in detail.

Change-Id: I0cffad1a3ee3ee86b61eca3c856476133125049b

solenv/gbuild/platform/macosx.mk

@@ -22,6 +22,8 @@ export VCL_HIDE_WINDOWS=1
 
 gb_SDKDIR := $(MACOSX_SDK_PATH)
 
+gb_COMPILEROPTFLAGS := -O1
+
 include $(GBUILDDIR)/platform/com_GCC_defs.mk
 
 # Darwin mktemp -t expects a prefix, not a pattern