qmake/vcxproj: Read C language standard from QMAKE_CFLAGS

The vcxproj generator completely ignored QMAKE_CFLAGS and did only read
QMAKE_CXXFLAGS.  The msbuild-internal "cl compiler tool" contains the
flags for both, C and C++.  It is to risky to take all QMAKE_CFLAGS into
account for the "cl compiler tool", because this might collide with what
is specified in QMAKE_CXXFLAGS.  Therefore, we only read the
/std:... compiler option from QMAKE_CFLAGS and set the
LanguageStandard_C flag in the msbuild file.

Task-number: QTBUG-89296
Change-Id: I885061802c1350b293a7868d4c9a9367d30e2380
Reviewed-by: Alexandru Croitor <[email protected]>
(cherry picked from commit a6a216e)

qmake/generators/win32/msvc_vcproj.cpp

@@ -37,6 +37,7 @@
 #include <qcryptographichash.h>
 #include <qhash.h>
 #include <quuid.h>
+#include <qregexp.h>
 
 #include <stdlib.h>
 
@@ -1052,6 +1053,20 @@ void VcprojGenerator::initConfiguration()
     initPreLinkEventTools();
 }
 
+// Filter from the given QMAKE_CFLAGS the options that are relevant
+// for the vcxproj-global VCCLCompilerTool.
+static ProStringList relevantCFlags(const ProStringList &flags)
+{
+    ProStringList result;
+    static const QRegExp rex("^[/-]std:.*");
+    for (const ProString &flag : flags) {
+        if (rex.exactMatch(flag.toQString())) {
+            result.append(flag);
+        }
+    }
+    return result;
+}
+
 void VcprojGenerator::initCompilerTool()
 {
     QString placement = project->first("OBJECTS_DIR").toQString();
@@ -1074,6 +1089,7 @@ void VcprojGenerator::initCompilerTool()
         conf.compiler.ForcedIncludeFiles       = project->values("PRECOMPILED_HEADER").toQStringList();
     }
 
+    conf.compiler.parseOptions(relevantCFlags(project->values("QMAKE_CFLAGS")));
     conf.compiler.parseOptions(project->values("QMAKE_CXXFLAGS"));
 
     if (project->isActiveConfig("windows"))