Force definition of some conditionally defined parts of <stdint.h>.

Force definition of some conditionally defined parts of <stdint.h>.

Got some errors about undefined 'UINT32_MAX' and others while trying
to build re2c with MINGW. These variables are defined conditionally in
<stdint.h> depending on the following defines:
    __STDC_LIMIT_MACROS
    __STDC_CONSTANT_MACROS
Fix: defined these symbols in the beginning of "src/util/c99_stdint.h".

Also found out that "src/util/c99_stdint.h" didn't compile in the
absense of system <stdint.h> (which is exactly the case when it was
written for). THe problem was caused by incorrect use of 'typedef'
statement; fixed easily. What a shame I didn't compile it before ;D