Check for GLES3 is incomplete

The recently added support for optional GLES in 92a523d2 is incomplete. This check essentially checks that the version number for glesv2 is >= 3. This boils down to the Version number in glesv2.pc somewhere on the filesystem.

While this works for mesa, this fails when building against Vivante's GLES implementation used by NXP or the implementation used by TI. The version provided in the TI package is 19.1.6, so greater equal than 3, while actually not providing GLES3/gl3.h, so compilation fails accordingly in a yocto build

../git/cube-shadertoy.c:37:10: fatal error: GLES3/gl3.h: No such file or directory
   37 | #include <GLES3/gl3.h>

NXP's version is 8.0 while actually providing GLES3/gl3.h, so this happens to build.

Relying on the package version providing glesv2.pc seems incorrect to me. The package version has no indication about supported API versions. So meson should check for the existence of GLES3/gl3.h instead.