openexr: Fix compilation with OpenEXR 2.4

It uses modern C++; adding -std=c++98 breaks the build.
19 jobs for backport-mr-807-into-1.16-openexr in 1 minute and 46 seconds (queued for 12 seconds)
Status Job ID Name Coverage
  Preparation
passed #876053
gst indent

00:00:37

passed #876052
manifest

00:00:59

 
  Build
canceled #876062
build cerbero android universal

canceled #876063
build cerbero cross win32

canceled #876064
build cerbero cross win64

canceled #876061
build cerbero fedora x86_64

canceled #876066
gst-ios-12.2
build cerbero ios universal

00:00:42

canceled #876065
gst-macos-10.14
build cerbero macos x86_64

00:00:42

canceled #876054
build fedora x86_64

00:00:47

canceled #876060
docker windows 1607
build msys2

canceled #876055
build nodebug fedora x86_64

canceled #876056
build static fedora x86_64

canceled #876057
build static nodebug fedora x86_64

canceled #876058
docker windows 1607
build vs2017 amd64

canceled #876059
docker windows 1607
build vs2017 x86

 
  Test
canceled #876067
check fedora

canceled #876068
integration testsuites fedora

canceled #876069
valgrind bad

 
  Integrate
canceled #876070
android universal examples