Add some more documentation, and print PLATFORM_OVERRIDE info during run

Makefile

@@ -11,7 +11,7 @@ include Makefile.conf
 export YML_PATH=$(YML2_PATH)
 
 .PHONY: all
-all:
+all: _platform_override_info
 	$(MAKE) -C asn.1 generate
 	$(MAKE) -C asn.1
 	$(MAKE) -C sync
@@ -51,3 +51,13 @@ package: clean
 .PHONY: db
 db:
 	$(MAKE) -C db db
+
+.PHONY: _platform_override
+ifdef PLATFORM_OVERRIDE
+_platform_override_info:
+	@echo "================================================"
+	@echo "PLATFORM_OVERRIDE is set to '$(PLATFORM_OVERRIDE)'."
+	@echo "================================================"
+else
+_platform_override_info:
+endif

build-config/common.conf

@@ -4,6 +4,7 @@
 # see LICENSE.txt
 
 # General remark: to use (only) system libraries, set all the *_INC and *_LIB variables to the empty string.
+# All the *_INC and *_LIB variables are command line flags, not paths. Thus, all *_INC variables' values must start with "-I", and all *_LIB variables' values must start with "-L".
 
 ######### General #########
 BUILD_ON:=$(shell uname)