bump OS X version

This increases the required version of OS X from 10.9 to 10.15, which is the
oldest version of OS X that Apple still supplies with security fixes.

Author: kroening

src/common

@@ -34,9 +34,9 @@ endif
   CP_CFLAGS = -MMD -MP
   CXXFLAGS ?= -Wall -O2
 ifeq ($(filter-out OSX OSX_Universal,$(BUILD_ENV_)),)
-  CP_CXXFLAGS += -MMD -MP -mmacosx-version-min=10.9 -std=c++11 -stdlib=libc++
-  LINKFLAGS += -mmacosx-version-min=10.9 -stdlib=libc++
-  LINKNATIVE += -mmacosx-version-min=10.9 -stdlib=libc++
+  CP_CXXFLAGS += -MMD -MP -mmacosx-version-min=10.15 -std=c++11 -stdlib=libc++
+  LINKFLAGS += -mmacosx-version-min=10.15 -stdlib=libc++
+  LINKNATIVE += -mmacosx-version-min=10.15 -stdlib=libc++
 else
   CP_CXXFLAGS += -MMD -MP -std=c++11
 endif