m4/pcre2_visibility.m4: make sure PCRE2_EXPORT is always safe (#413)

Since a8a875e (windows: integrate visibility attributes without
conflicts (#249), 2023-05-16), PCRE2_EXPORT was meant to be
defined as an empty value for compilers that don't support
-fvisibility in *NIX, but a branch was left uncovered by mistake
resulting in compilation issues with propietary compilers in at
least Solaris and AIX.

define PCRE2_EXPORT even if the compiler is not GCC compatible.

ChangeLog

@@ -55,6 +55,8 @@ misunderstanding one of the grapheme sequence breaking rules in Unicode Annex
 break property unless a zero-width joiner intervenes. PCRE2 was not insisting
 on the ZWJ, causing \X to match more than it should. See GitHub issue #410.
 
+8. Avoid compilation issues with propietary compilers in UNIX since 10.43.
+
 
 Version 10.43 16-February-2024
 ------------------------------

m4/pcre2_visibility.m4

@@ -77,6 +77,8 @@ AC_DEFUN([PCRE2_VISIBILITY],
     else
       AC_DEFINE(PCRE2_EXPORT, [], [to make a symbol visible])
     fi
+  else
+    AC_DEFINE(PCRE2_EXPORT, [], [to make a symbol visible])
   fi
   AC_SUBST([VISIBILITY_CFLAGS])
   AC_SUBST([VISIBILITY_CXXFLAGS])