Added test for mixed C/C++ pch usage.

test cases/15 mixed pch/builder.txt

@@ -0,0 +1,4 @@
+project('mixed C and C++ pch test', 'c++', 'c')
+
+exe = executable('prog', 'main.cc', 'func.c')
+exe.pch('pch/main.hh', 'pch/func.h')

test cases/15 mixed pch/func.c

@@ -0,0 +1,7 @@
+void tmp_func() {
+    fprintf(stdout, "This is a function that fails if stdio is not #included.\n");
+}
+
+int cfunc() {
+    return 0;
+}

test cases/15 mixed pch/main.cc

@@ -0,0 +1,10 @@
+extern "C" int cfunc();
+
+void func() {
+    std::cout << "This is a function that fails to compile if iostream is not included."
+              << std::endl;
+}
+
+int main(int argc, char **argv) {
+    return cfunc();
+}

test cases/15 mixed pch/pch/func.h

@@ -0,0 +1 @@
+#include<stdio.h>

test cases/15 mixed pch/pch/main.hh

@@ -0,0 +1 @@
+#include<iostream>