Makefile.Libsupport: add more customization options

This change allows the customization of the exit mechanism in the last
chance handler, and to pass compilation flags when building the support
library.

This is in preparation of the introduction of a CCG testsuite run.
The heuristics to choose which compilation flags need to be passed to
gprbuild, and which exit mechanism needs to be used in the last chance handler
do not work in a CCG context. Instead of adding corner cases for each
configuration, allow configuring these features during the make invocation.

Author: leocreuse

tools/gnatcov/examples/support/Makefile.libsupport

@@ -7,7 +7,7 @@ include Makefile.common
 # at this stage.
 
 ifneq ($(TARGET),)
-LIBSUPPORT_CFLAGS=-fdump-scos -fpreserve-control-flow -g
+LIBSUPPORT_CFLAGS:=-fdump-scos -fpreserve-control-flow -g $(LIBSUPPORT_CFLAGS)
 endif
 
 all: $(SUPPORT_TARGET_SRC_DIR) $(LIBSUPPORT_DIR) LIBSUPPORT LCH
@@ -21,7 +21,7 @@ all: $(SUPPORT_TARGET_SRC_DIR) $(LIBSUPPORT_DIR) LIBSUPPORT LCH
 # Pick a default kind of silent last chance handler, that users of this
 # Makefile can override if needed, e.g. for VxWorks kinds of configurations
 # which are cross targets with "full" runtimes akin to native profiles.
-SILENT_LCH = $(strip $(if $(TARGET), abort, exit))
+SILENT_LCH ?= $(strip $(if $(TARGET), abort, exit))
 
 conf.gpr: force
 	sed -e 's/%TARGET%/$(TARGET)/g' \