build: Do not set CFLAGS/LDFLAGS directly

build: Do not set CFLAGS/LDFLAGS directly

Set a separate variable for adding warning flags, optimization, etc.
Build systems are not supposed to change CFLAGS and LDFLAGS, these are
user variables.

Doing so we guarantee CFLAGS and LDFLAGS from environment is appended
to the flags used during build. One useful use-case is to temporarily
disable -Werror when using --enable-maintainer-mode, without completely
loosing the warning flags and other parameters in CFLAGS (like -fPIC).

Without this patch, fiddling with CFLAGS/LDFLAGS after configure may
result in errors like below:

/usr/bin/ld: tools/rfcomm.o: relocation R_X86_64_32 against `.bss' can
not be used when making a shared object; recompile with -fPIC
tools/rfcomm.o: could not read symbols: Bad value
collect2: error: ld returned 1 exit status
make[1]: *** [tools/rfcomm] Error 1
make: *** [all] Error 2

Reference: http://www.gnu.org/software/automake/manual/html_node/Flag-Variables-Ordering.html