alog2 defined in multiple code files

[edwardhartnett]

This constant should be defined as a pre-processor symbol in grib2.h.

(base) ed@mikado:~/NCEPLIBS-g2c/b/tests$ find ../../src -name '*.c'|xargs grep 'alog2 ='
../../src/simpack.c:    static g2float alog2 = 0.69314718;       /*  ln(2.0) */
../../src/pngpack.c:    static g2float alog2 = 0.69314718;       /*  ln(2.0) */
(base) ed@mikado:~/NCEPLIBS-g2c/b/tests$ find ../../src -name '*.c'|xargs grep 'alog2='
../../src/compack.c:    static g2float alog2=0.69314718;       //  ln(2.0)
../../src/misspack.c:    static g2float alog2=0.69314718;       //  ln(2.0)
../../src/jpcpack.c:    static g2float alog2=0.69314718;       //  ln(2.0)