Why do you believe this to be the case?  A quick grep doesn't show the Xmu headers referenced from any of the mesa headers.

I was building a friend's code when I was missing headers. Asked what was needed and I had everything. It turned out that libXmu-devel was missing. Either way, Gentoo has the following (unless they have unneeded dependencies here or parts are disabled in Fedora):

$ paludis -qD mesa
* media-libs/mesa
    gentoo:                  6.5.2-r1 7.0.3(~) 7.1_rc3(~) 7.1(~) 7.2_rc1(~) 7.2(~) (7.3)R(~) {:0} 
    layman:                  (7.0.3 (in ::pda))X (9999 (in ::THE))X (9999-r1 (in ::nouveau))X (9999-r2 (in ::nouveau))X {:0} 
    installed:               7.2* {:0} 
    Description:             OpenGL-like graphic library for Linux
    Homepage:                http://mesa3d.sourceforge.net/
    License:                 LGPL-2
    Installed time:          Sun Jan 25 20:25:05 2009
    Use flags:               (-debug) (doc) (-motif) (nptl) (-pic) (xcb) video_cards: (-intel) (-mach64) (-mga) (-none) (-r128) (-radeon) (-s3virge) (-savage) (-sis) (-sunffb) (-tdfx) (-trident) (-via)
    From repositories:       gentoo
    Installed using:         paludis-0.34.0
    Build dependencies:      
            >=app-admin/eselect-opengl-1.0.6-r1
            dev-libs/expat
            >=x11-libs/libX11-1.1.5
            >=x11-libs/libXext-1.0.4
            >=x11-libs/libXxf86vm-1.0.2
            >=x11-libs/libXi-1.1.3
            >=x11-libs/libXmu-1.0.3
            >=x11-libs/libXdamage-1.1.1
            >=x11-libs/libdrm-2.3.1
            >=x11-libs/libICE-1.0.4
            motif? (
                >=x11-libs/openmotif-2.3.1-r1
            )
            doc? (
                >=app-doc/opengl-manpages-20001215
            )
            !<=x11-base/xorg-x11-6.9
            !<=x11-proto/xf86driproto-2.0.3
            dev-util/pkgconfig
            >=x11-misc/makedepend-1.0.1
            >=x11-proto/inputproto-1.4.4
            >=x11-proto/xextproto-7.0.3
            !hppa? (
                >=x11-proto/xf86driproto-2.0.4
            )
            >=x11-proto/xf86vidmodeproto-2.2.2
            >=x11-proto/glproto-1.4.9
            motif? (
                >=x11-proto/printproto-1.0.4
            )
            =sys-devel/automake-1.10*
            >=sys-devel/autoconf-2.61
            sys-devel/libtool
    Run dependencies:        
            >=app-admin/eselect-opengl-1.0.6-r1
            dev-libs/expat
            >=x11-libs/libX11-1.1.5
            >=x11-libs/libXext-1.0.4
            >=x11-libs/libXxf86vm-1.0.2
            >=x11-libs/libXi-1.1.3
            >=x11-libs/libXmu-1.0.3
            >=x11-libs/libXdamage-1.1.1
            >=x11-libs/libdrm-2.3.1
            >=x11-libs/libICE-1.0.4
            motif? (
                >=x11-libs/openmotif-2.3.1-r1
            )
            doc? (
                >=app-doc/opengl-manpages-20001215
            )
            !<=x11-base/xorg-x11-6.9
    Post dependencies:       
    Source URI:              mirror://sourceforge/mesa3d/MesaLib-7.2.tar.bz2
 
Key to mask reasons:
 
* R: repository
* X: unavailable
* ~: keyword (unstable accepted)

Sounds like your friend's code needs libXmu-devel then.  None of the mesa headers reference libXmu headers, even transitively.