more compiler flags when --enable-debug

[bgoglin]

It might be good to add more compiler flags (warnings, etc) to CFLAGS when --enable-debug is passed to configure.

We currently add
-Wall -Wunused-parameter -Wundef -Wno-long-long -Wsign-compare -Wmissing-prototypes -Wstrict-prototypes -Wcomment -pedantic -Wshadow

For instance -Wshorten64-to-32 (#130), -Wformat=2 -Wformat-signedness (cppcheck used to report many such warnings).

We may want to check them during configure in case they are not supported by non-gcc compilers. icc does not support -Wformat-signedness, it warns and ignores it without failing.

[bgoglin]

config/hwloc.m4 now has _HWLOC_CHECK_CC_OPTION() to enable debug flags only if supported by the compiler, so this issue should be fairly easy if more flags are still needed.