Use -fmacro-prefix-map for utils

The __FILE__ macro is used in the EXCEPTION macro to report the file
name alongside error messages. This macro reports the file name
exactly as it is passed to the compiler. For most source files in
libgourou this is a nice relative path such as "src/libgourou.cpp".
However, for EXCEPTION instances in libgourou_common.h, the compiler
flag is "-I$(ROOT)/include", where $(ROOT) is an absolute path passed
from the higher Makefile. This results in an absolute path to the
build directory being hardcoded into the utils shared library and
binaries, and reported in error messages. Besides being less readable
than the more common relative paths, this triggers warnings from
packaging tools that detect inadvertent references to temporary build
directories that end up in compiled binaries and might indicate a bug.

There is a GCC feature -fmacro-prefix-map which allows to perform
substition on the values that are used for __FILE__. Use that option
to strip out the absolute path component, without changing any
functionality.

The feature was added to GCC on 2018-01-18 and released in GCC 8.1.0.

7365279fca

utils/Makefile

@@ -6,7 +6,7 @@ TARGETS=$(TARGET_BINARIES) launcher
 
 MAN_PAGES=acsmdownloader adept_activate adept_remove adept_loan_mgt
 
-CXXFLAGS=-Wall -fPIC -I$(ROOT)/include -fdata-sections -ffunction-sections
+CXXFLAGS=-Wall -fPIC -I$(ROOT)/include -fmacro-prefix-map=$(ROOT)/= -fdata-sections -ffunction-sections
 
 STATIC_DEP=
 # LDFLAGS += -Wl,--gc-sections