With the release of GL_ARB_ES2_compatibility, a GL desktop context supports ignoring the precision qualifiers in a GLSL shader. thus adding the defines:

"#define lowp\n"
"#define mediump\n"
"#define highp\n";

is not a good idea. Cards supporting GL_ARB_ES2_compatibility include GeForce 6 all the way to GeForce 4xx (with recent drivers). Additionally we should be seeing ATI drivers with this extension very, very soon [it is core in GL4.1 but does not require GL4 hardware] in corresponding generation of cards. [for the record GeForce 6 was first released April 2004, over 6 years ago].

Additionally adding those defines does not in any way handle "default precision qualifiers in GLSL for GLES2", see section 4.5.3 of "The Open GL ES shading Language", http://www.khronos.org/registry/gles/specs/2.0/GLSL_ES_Specification_1.0.17.pdf