Add missing void argument to function prototypes.
Accept constant string input for creating recordsets.

Add const qualifier to the arguments of ihex_rs_from_file() and
ihex_rs_from_string() to ease library usage.  These functions never
need to modify their arguments.
Rename CCFLAGS make variable to standard CFLAGS.
Check for errors when allocating record data memory.

The function ihex_parse_single_record() did not check the return value
of malloc() system call when allocating memory for the record's data.
It now sets IHEX_ERR_MALLOC_FAILED and aborts in that case.
Avoid linker errors with inline function and -O0 optimization.

GCC reported a linker error because of a missing ihex_fromhex4()
function when compiling without optimization.  Since the function is
only used in ihex_parse.c and declared inline, it may as well be
declared static.
Remove unused variable in test_memory_is_copied_1().
Fix misspelled type name u_int8_t in tests.
Mark unused variables in tests to squelch compiler warnings.

For convenience and compatibility, define a macro UNUSED with the
actual attribute declaration.
Fix missing cast in ihex_fromhex*() tests.

The tested functions expect pointers to uint8_t data, but the given
string literals are by default (const char*), which is signed.
Add missing void argument to function declarations.
Merge pull request #9 from acolomb/cleanup

Code cleanup and minor error corrections