Convert public char* to const char* for C++ compatibility. #116

Job	Run time
Cmake (gcc)	1m 20s
Cmake (gcc, -DBUILD_EXAMPLES=ON -DBUILD_TESTS=ON)	1m 31s
Cmake (gcc, -DUSE_DTOA_LIBRARY=ON -DBUILD_TESTS=ON)	1m 30s
Cmake (gcc, -DUSE_MEM_FILE=ON -DBUILD_TESTS=ON)	1m 25s
Cmake (gcc, -DUSE_NO_MD5=ON -DBUILD_TESTS=ON)	1m 51s
Cmake (gcc, -DUSE_OPENSSL_MD5=ON -DBUILD_TESTS=ON)	1m 51s
Cmake (gcc, -DUSE_STANDARD_TMPFILE=ON -DBUILD_TESTS=ON)	1m 24s
Cmake (gcc, -DUSE_SYSTEM_MINIZIP=ON -DBUILD_TESTS=ON)	1m 18s
Cmake (gcc, -DUSE_SYSTEM_MINIZIP=ON -DUSE_OPENSSL_MD5=ON -DBUILD_TESTS=ON)	1m 48s
Cmake (clang)	2m 21s
Cmake (clang, -DBUILD_EXAMPLES=ON -DBUILD_TESTS=ON)	1m 46s
Cmake (clang, -DUSE_DTOA_LIBRARY=ON -DBUILD_TESTS=ON)	1m 49s
Cmake (clang, -DUSE_MEM_FILE=ON -DBUILD_TESTS=ON)	2m 20s
Cmake (clang, -DUSE_NO_MD5=ON -DBUILD_TESTS=ON)	1m 43s
Cmake (clang, -DUSE_OPENSSL_MD5=ON -DBUILD_TESTS=ON)	2m 26s
Cmake (clang, -DUSE_STANDARD_TMPFILE=ON -DBUILD_TESTS=ON)	2m 0s
Cmake (clang, -DUSE_SYSTEM_MINIZIP=ON -DBUILD_TESTS=ON)	1m 32s
Cmake (clang, -DUSE_SYSTEM_MINIZIP=ON -DUSE_OPENSSL_MD5=ON -DBUILD_TESTS=ON)	2m 28s
	32m 23s

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