PCRE2: Explicitly specify cmake c standard

* Fixes compilation issues with for loops

recipes/pcre2/all/conanfile.py

@@ -98,6 +98,8 @@ def generate(self):
         tc.variables["PCRE2_BUILD_PCRE2_32"] = self.options.build_pcre2_32
         tc.variables["PCRE2_SUPPORT_JIT"] = self.options.support_jit
         tc.variables["PCRE2GREP_SUPPORT_CALLOUT_FORK"] = self.options.get_safe("grep_support_callout_fork", False)
+        # Fix for error: 'for' loop initial declarations are only allowed in C99 or C11 mode
+        tc.variables["CMAKE_C_STANDARD"] = "99"
         if Version(self.version) < "10.38":
             # relocatable shared libs on Macos
             tc.cache_variables["CMAKE_POLICY_DEFAULT_CMP0042"] = "NEW"