configure: Avoid deprecated AC_PROG_CC_C99 if possible. #1839

Job	Run time
osx clang --disable-ssl	3m 49s
linux deb no dpdk	9m 25s
linux deb shared dpdk	15m 34s
linux gcc --disable-ssl	4m 2s
linux clang --disable-ssl	3m 2s
linux gcc c99	4m 13s
linux clang c99	3m 49s
linux gcc test	11m 57s
linux clang test asan	18m 30s
linux clang test ubsan	10m 45s
linux gcc test --enable-shared	14m 10s
linux clang test --enable-shared	14m 42s
linux gcc test dpdk	11m 3s
linux clang test dpdk	10m 34s
linux gcc test -ljemalloc	12m 43s
linux clang test -ljemalloc	11m 6s
linux gcc afxdp 5.3	4m 14s
linux clang afxdp 5.3	3m 59s
linux gcc dpdk --enable-shared	5m 29s
linux clang dpdk --enable-shared	4m 59s
linux gcc dpdk-shared	4m 1s
linux clang dpdk-shared	3m 57s
linux gcc dpdk-shared --enable-shared	4m 50s
linux clang dpdk-shared --enable-shared	4m 26s
linux gcc m32 --disable-ssl	3m 21s
	3h 18m 40s

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