MIPS: Fixed undefined MSA interfaces

[XiWeiGu]

When compiling on the MIPS platform using the following command: ./configure --enable-hardware-optimizations && make The options -mmsa -mfp64 are not being passed. PNG_MIPS_MSA_IMPLEMENTATION is defined as 2, leading to the initialization of unimplemented MSA interfaces. Error message：

libtool: link: gcc -g -O2 -o .libs/pngfix contrib/tools/pngfix.o ./.libs/libpng16.so -lz -lm /usr/bin/ld: ./.libs/libpng16.so: undefined reference to png_read_filter_row_avg4_msa' /usr/bin/ld: ./.libs/libpng16.so: undefined reference topng_read_filter_row_sub3_msa' /usr/bin/ld: ./.libs/libpng16.so: undefined reference to png_read_filter_row_up_msa' /usr/bin/ld: ./.libs/libpng16.so: undefined reference topng_read_filter_row_avg3_msa' /usr/bin/ld: ./.libs/libpng16.so: undefined reference to png_read_filter_row_sub4_msa' /usr/bin/ld: ./.libs/libpng16.so: undefined reference topng_read_filter_row_paeth3_msa' /usr/bin/ld: ./.libs/libpng16.so: undefined reference to `png_read_filter_row_paeth4_msa' collect2: error: ld returned 1 exit status