configure.ac: don't use == with test
configure.ac: don't use == with test

Although it works, it's not the correct thing to do.

v2: Rebase
v3: Rebase

Signed-off-by: Emil Velikov <f20878473f414da183da19ee25c8722cc1d711e4@collabora.com>
Reviewed-by: Tobias Droste <6f0b7afb8e169bccf102c27c4328261728913687@gmx.de> (v1)

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler