Omit -fPIC if shared library build is disabled. (#732)

Details:
- Updated common.mk so that when --disable-shared option is given to
  configure:
  1. The -fPIC compiler flag is omitted from the individual 
     configuration family members' CPICFLAGS variables (which are 
     initialized in each subconfig's make_defs.mk file); and
  2. The BUILD_SYMFLAGS variable, which contains compiler flags needed 
     to control the symbol export behavior, is left blank.
- The net result of these changes is that flags specific to shared
  library builds are only used when a shared library is actually 
  scheduled to be built. Thanks to Nick Knight for reporting this issue.
- CREDITS file update.

CREDITS

@@ -56,6 +56,7 @@ but many others have contributed code, ideas, and feedback, including
   Tony Kelman              @tkelman
   Lee Killough             @leekillough               (Cray)
   Mike Kistler             @mkistler                  (IBM, Austin Research Laboratory)
+  Nick Knight              @nick-knight               (SiFive)
   Ivan Korostelev          @ivan23kor                 (University of Alberta)
   Kyungmin Lee             @kyungminlee               (Ohio State University)
   Michael Lehn             @michael-lehn

common.mk

@@ -731,6 +731,9 @@ $(foreach c, $(CONFIG_LIST_FAM), $(eval $(call append-var-for,CWARNFLAGS,$(c))))
 
 # --- Position-independent code flags (shared libraries only) ---
 
+
+ifeq ($(MK_ENABLE_SHARED),yes)
+
 # Emit position-independent code for dynamic linking.
 ifeq ($(IS_MSVC),yes)
 # Note: Don't use any fPIC flags for Windows builds since all code is position-
@@ -814,6 +817,14 @@ endif
 endif
 endif
 
+else #ifeq ($(MK_ENABLE_SHARED),no)
+
+# Don't modify CPICFLAGS for the various configuration family members.
+# Don't use any special symbol export flags.
+BUILD_SYMFLAGS :=
+
+endif
+
 # --- Language flags ---
 
 # Enable C99.