Error messages when compiling postgres6.5

Below is what I encountered when compiling postgres6.5...
My kernel version is slakware2.0.35 and my system got pg1.09
installed now.(But I have killed the process "postmaster")

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gcc -I../../include -I../../backend -O2 -Wall -Wmissing-prototypes
-I..
-c
stringinfo.c -o stringinfo.o
stringinfo.c: In function `appendStringInfo':
stringinfo.c:115: `va_list' undeclared (first use this function)
stringinfo.c:115: (Each undeclared identifier is reported only once
stringinfo.c:115: for each function it appears in.)
stringinfo.c:115: parse error before `args'
stringinfo.c:121: warning: implicit declaration of function `va_start'
stringinfo.c:121: `args' undeclared (first use this function)
stringinfo.c:123: warning: implicit declaration of function `va_end'
make[2]: *** [stringinfo.o] Error 1
make[2]: Leaving directory `/usr/src/postgresql-6.5/src/backend/lib'
make[1]: *** [lib.dir] Error 2
make[1]: Leaving directory `/usr/src/postgresql-6.5/src/backend'
make: *** [all] Error 2

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What is the problem actually..:? (gcc compiler version?...)

Thanks very much for your help..