Revert "Select nonstd string_view via CXX_FLAGS (nonstd-lite-project issue 42)"

This reverts commit f7c2e7e.

Author: martinmoene

appveyor.yml

@@ -20,9 +20,9 @@ build:
 environment:
     matrix:
         - generator: "Visual Studio 15 2017"
-          select_sv: -DCMAKE_CXX_FLAGS:="-Dstring_view_CONFIG_SELECT_STRING_VIEW=string_view_STRING_VIEW_STD"
+          select_sv: -DSTRING_VIEW_LITE_OPT_SELECT_STD=ON
         - generator: "Visual Studio 15 2017"
-          select_sv: -DCMAKE_CXX_FLAGS:="-Dstring_view_CONFIG_SELECT_STRING_VIEW=string_view_STRING_VIEW_NONSTD"
+          select_sv: -DSTRING_VIEW_LITE_OPT_SELECT_NONSTD=ON
         - generator: "Visual Studio 14 2015"
         - generator: "Visual Studio 12 2013"
         - generator: "Visual Studio 11 2012"
@@ -49,7 +49,7 @@ matrix:
 
 before_build:
     - mkdir build && cd build
-    - cmake -A %platform% -G "%generator%" "%select_sv%" -DCMAKE_CXX_FLAGS:="-Dstring_view_CONFIG_SELECT_STRING_VIEW=string_view_STRING_VIEW_NONSTD" -DSTRING_VIEW_LITE_OPT_BUILD_TESTS=ON -DSTRING_VIEW_LITE_OPT_BUILD_EXAMPLES=OFF ..
+    - cmake -A %platform% -G "%generator%" "%select_sv%" -DSTRING_VIEW_LITE_OPT_BUILD_TESTS=ON -DSTRING_VIEW_LITE_OPT_BUILD_EXAMPLES=OFF ..
 
 build_script:
     - cmake --build . --config %configuration%