Use the C library version of log10() instead of the inaccurate formula

log10(x) = log10e * log(x).  The formula would work if the RHS were
evaluated in extended precision with an extended precision log().
This actually happened with the i387 log() because it returns excess
precision.

Found by:	ucbtest

lib/libF77/r_lg10.c

@@ -1,7 +1,5 @@
 #include "f2c.h"
 
-#define log10e 0.43429448190325182765
-
 #ifdef KR_headers
 double log();
 double r_lg10(x) real *x;
@@ -11,5 +9,5 @@ double r_lg10(x) real *x;
 double r_lg10(real *x)
 #endif
 {
-return( log10e * log(*x) );
+return( log10(*x) );
 }