Clear CFLAGS and CXXFLAGS before tests (#472)

Some test cases check that a compiler flag is not present.  But
cc::Build loads additional flags from the CFLAGS and CXXFLAGS
environment variables.  If these are set, they might interfere with the
test cases.

Therefore we clear the CFLAGS and CXXFLAGS environment variables before
running a test that requires an absent flag.

tests/test.rs

@@ -2,8 +2,18 @@ use crate::support::Test;
 
 mod support;
 
+// Some tests check that a flag is *not* present.  These tests might fail if the flag is set in the
+// CFLAGS or CXXFLAGS environment variables.  This function clears the CFLAGS and CXXFLAGS
+// variables to make sure that the tests can run correctly.
+fn reset_env() {
+    std::env::set_var("CFLAGS", "");
+    std::env::set_var("CXXFLAGS", "");
+}
+
 #[test]
 fn gnu_smoke() {
+    reset_env();
+
     let test = Test::gnu();
     test.gcc().file("foo.c").compile("foo");
 
@@ -19,6 +29,8 @@ fn gnu_smoke() {
 
 #[test]
 fn gnu_opt_level_1() {
+    reset_env();
+
     let test = Test::gnu();
     test.gcc().opt_level(1).file("foo.c").compile("foo");
 
@@ -27,6 +39,8 @@ fn gnu_opt_level_1() {
 
 #[test]
 fn gnu_opt_level_s() {
+    reset_env();
+
     let test = Test::gnu();
     test.gcc().opt_level_str("s").file("foo.c").compile("foo");
 
@@ -56,6 +70,8 @@ fn gnu_debug_fp() {
 
 #[test]
 fn gnu_debug_nofp() {
+    reset_env();
+
     let test = Test::gnu();
     test.gcc()
         .debug(true)
@@ -100,6 +116,8 @@ fn gnu_warnings() {
 
 #[test]
 fn gnu_extra_warnings0() {
+    reset_env();
+
     let test = Test::gnu();
     test.gcc()
         .warnings(true)
@@ -113,6 +131,8 @@ fn gnu_extra_warnings0() {
 
 #[test]
 fn gnu_extra_warnings1() {
+    reset_env();
+
     let test = Test::gnu();
     test.gcc()
         .warnings(false)
@@ -126,6 +146,8 @@ fn gnu_extra_warnings1() {
 
 #[test]
 fn gnu_warnings_overridable() {
+    reset_env();
+
     let test = Test::gnu();
     test.gcc()
         .warnings(true)
@@ -154,6 +176,8 @@ fn gnu_x86_64() {
 
 #[test]
 fn gnu_x86_64_no_pic() {
+    reset_env();
+
     for vendor in &["unknown-linux-gnu", "apple-darwin"] {
         let target = format!("x86_64-{}", vendor);
         let test = Test::gnu();
@@ -215,6 +239,8 @@ fn gnu_x86_64_no_plt() {
 
 #[test]
 fn gnu_set_stdlib() {
+    reset_env();
+
     let test = Test::gnu();
     test.gcc()
         .cpp_set_stdlib(Some("foo"))
@@ -253,6 +279,8 @@ fn gnu_compile_assembly() {
 
 #[test]
 fn gnu_shared() {
+    reset_env();
+
     let test = Test::gnu();
     test.gcc()
         .file("foo.c")
@@ -265,6 +293,8 @@ fn gnu_shared() {
 
 #[test]
 fn gnu_flag_if_supported() {
+    reset_env();
+
     if cfg!(windows) {
         return;
     }
@@ -301,6 +331,8 @@ fn gnu_flag_if_supported_cpp() {
 
 #[test]
 fn gnu_static() {
+    reset_env();
+
     let test = Test::gnu();
     test.gcc()
         .file("foo.c")
@@ -313,6 +345,8 @@ fn gnu_static() {
 
 #[test]
 fn msvc_smoke() {
+    reset_env();
+
     let test = Test::msvc();
     test.gcc().file("foo.c").compile("foo");
 
@@ -327,6 +361,8 @@ fn msvc_smoke() {
 
 #[test]
 fn msvc_opt_level_0() {
+    reset_env();
+
     let test = Test::msvc();
     test.gcc().opt_level(0).file("foo.c").compile("foo");