argyllcms: Fix compilation with GCC 10

A common mistake in C is omitting extern when declaring a global variable
in a header file.  If the header is included by several files it  results
in multiple definitions of the same variable.  In previous GCC versions
this error is ignored.  GCC 10 defaults to -fno-common,  which means a
linker error will now be reported. To fix this, use extern in header
files when declaring global variables, and ensure each global is defined
in exactly one C file. As a workaround, legacy C code can be compiled
with -fcommon.

      int x;  // tentative definition - avoid in header files 
      extern int y;  // correct declaration in a header file 

References:
https://bugzilla.opensuse.org/show_bug.cgi?id=1160244
https://bugzilla.opensuse.org/show_bug.cgi?id=1160256

In case of vinflate.c and inflate.c, both files define local variables
with the same name as global. It is no more possible with GCC 10. To
prevent sharing variables across files, add "static" to all local
variables.

Index: Argyll_V2.1.2/gamut/gamut.h
===================================================================
--- Argyll_V2.1.2.orig/gamut/gamut.h
+++ Argyll_V2.1.2/gamut/gamut.h
@@ -36,7 +36,7 @@
 #define MAXGAMN 10				/* Maximum gamut point neighbors returned */
 #define NSLOTS 6				/* Number of maximum direction slots */
 
-struct _vrml *wrl;				/* Declared in vrml.h, which may be #included after this */
+struct _vrml;					/* Declared in vrml.h, which may be #included after this */
 
 /* ------------------------------------ */
 #define NODE_STRUCT							\
Index: Argyll_V2.1.2/spectro/vinflate.c
===================================================================
--- Argyll_V2.1.2.orig/spectro/vinflate.c
+++ Argyll_V2.1.2/spectro/vinflate.c
@@ -92,7 +92,7 @@ int vinflate(void);
 */
 
 #define WSIZE 0x8000
-unsigned int wp;             /* current position in slide */
+static unsigned int wp;             /* current position in slide */
 uch slide[32768];
 
 static int vflush_output(unsigned int w) {
@@ -160,8 +160,8 @@ static ush cpdext[] = {         /* Extra
    the stream.
  */
 
-ulg bb;                         /* bit buffer */
-unsigned bk;                    /* bits in bit buffer */
+static ulg bb;                         /* bit buffer */
+static unsigned bk;                    /* bits in bit buffer */
 
 ush vmask_bits[] = {
     0x0000,
@@ -230,8 +230,8 @@ ush vmask_bits[] = {
  */
 
 
-int vlbits = 9;          /* bits in base literal/length lookup table */
-int vdbits = 6;          /* bits in base distance lookup table */
+static int vlbits = 9;          /* bits in base literal/length lookup table */
+static int vdbits = 6;          /* bits in base distance lookup table */
 
 
 /* If BMAX needs to be larger than 16, then h and x[] should be ulg. */
@@ -239,7 +239,7 @@ int vdbits = 6;          /* bits in base
 #define N_MAX 288       /* maximum number of codes in any set */
 
 
-unsigned hufts;         /* track memory usage */
+static unsigned hufts;  /* track memory usage */
 
 /* Given a list of code lengths and a maximum table size, make a set of
    tables to decode that set of codes.  Return zero on success, one if
Index: Argyll_V2.1.2/spectro/inflate.c
===================================================================
--- Argyll_V2.1.2.orig/spectro/inflate.c
+++ Argyll_V2.1.2/spectro/inflate.c
@@ -49,7 +49,7 @@ typedef unsigned int ulg;
    the next table, which codes e - 16 bits, and lastly e == 99 indicates
    an unused code.  If a code with e == 99 is looked up, this implies an
    error in the data. */
-struct huft {
+static struct huft {
   uch e;                /* number of extra bits or operation */
   uch b;                /* number of bits in this code or subcode */
   union {
@@ -87,7 +87,7 @@ int inflate(void);
 /* unsigned wp;             current position in slide */
 
 #define WSIZE 0x8000
-unsigned int wp;             /* current position in slide */
+static unsigned int wp;             /* current position in slide */
 uch slide[32768];
 
 static int flush_output(unsigned int w) {
@@ -155,8 +155,8 @@ static ush cpdext[] = {         /* Extra
    the stream.
  */
 
-ulg bb;                         /* bit buffer */
-unsigned bk;                    /* bits in bit buffer */
+static ulg bb;                         /* bit buffer */
+static unsigned bk;                    /* bits in bit buffer */
 
 ush mask_bits[] = {
     0x0000,
@@ -201,8 +201,8 @@ ush mask_bits[] = {
  */
 
 
-int lbits = 9;          /* bits in base literal/length lookup table */
-int dbits = 6;          /* bits in base distance lookup table */
+static int lbits = 9;          /* bits in base literal/length lookup table */
+static int dbits = 6;          /* bits in base distance lookup table */
 
 
 /* If BMAX needs to be larger than 16, then h and x[] should be ulg. */
@@ -210,7 +210,7 @@ int dbits = 6;          /* bits in base
 #define N_MAX 288       /* maximum number of codes in any set */
 
 
-unsigned hufts;         /* track memory usage */
+static unsigned hufts;         /* track memory usage */
 
 
 static int huft_build(b, n, s, d, e, t, m)