What errors did you get without this patch?  Do other demos in progs/egl/ need to link to libm?

Without this patch, trigonometric functions fail to resolve. The other progs in that directory do not require it. Here is linker output:

gcc -march=x86-64 -mtune=generic -O2 -pipe -Wall -Wmissing-prototypes -std=c99 -ffast-math -fvisibility=hidden -fno-strict-aliasing  -fPIC  -DUSE_X86_64_ASM -D_GNU_SOURCE -DPTHREADS -DHAVE_POSIX_MEMALIGN -DUSE_XCB -DGLX_USE_TLS -DPTHREADS -DUSE_EXTERNAL_DXTN_LIB=1 -DIN_DRI_DRIVER -DGLX_DIRECT_RENDERING -DGLX_INDIRECT_RENDERING -DHAVE_ALIAS -DHAVE_XEXTPROTO_71 -Wl,--hash-style=gnu -Wl,--as-needed -o eglgears eglgears.o -L../../lib -lEGL -lGL -ldrm  
eglgears.o: In function `T.55':
eglgears.c:(.text+0x183): undefined reference to `sincos'
eglgears.c:(.text+0x25a): undefined reference to `sincos'
eglgears.c:(.text+0x312): undefined reference to `sincos'
eglgears.c:(.text+0x36b): undefined reference to `sincos'
eglgears.c:(.text+0x3c4): undefined reference to `sincos'
eglgears.o:eglgears.c:(.text+0x41a): more undefined references to `sincos' follow
eglgears.o: In function `T.55':
eglgears.c:(.text+0x854): undefined reference to `sincosf'
eglgears.c:(.text+0x88c): undefined reference to `sincos'
eglgears.c:(.text+0x91f): undefined reference to `sincos'
eglgears.c:(.text+0xa2f): undefined reference to `sincos'
eglgears.c:(.text+0xabc): undefined reference to `sincos'
eglgears.c:(.text+0xb06): undefined reference to `sin'
eglgears.c:(.text+0xc59): undefined reference to `sincosf'
eglgears.c:(.text+0xc93): undefined reference to `sincos'
collect2: ld returned 1 exit status
make: [eglgears] Error 1 (ignored)
gcc -march=x86-64 -mtune=generic -O2 -pipe -Wall -Wmissing-prototypes -std=c99 -ffast-math -fvisibility=hidden -fno-strict-aliasing  -fPIC  -DUSE_X86_64_ASM -D_GNU_SOURCE -DPTHREADS -DHAVE_POSIX_MEMALIGN -DUSE_XCB -DGLX_USE_TLS -DPTHREADS -DUSE_EXTERNAL_DXTN_LIB=1 -DIN_DRI_DRIVER -DGLX_DIRECT_RENDERING -DGLX_INDIRECT_RENDERING -DHAVE_ALIAS -DHAVE_XEXTPROTO_71 -Wl,--hash-style=gnu -Wl,--as-needed -o peglgears peglgears.o -L../../lib -lEGL -lGL -ldrm  
peglgears.o: In function `T.48':
peglgears.c:(.text+0x183): undefined reference to `sincos'
peglgears.c:(.text+0x25a): undefined reference to `sincos'
peglgears.c:(.text+0x312): undefined reference to `sincos'
peglgears.c:(.text+0x36b): undefined reference to `sincos'
peglgears.c:(.text+0x3c4): undefined reference to `sincos'
peglgears.o:peglgears.c:(.text+0x41a): more undefined references to `sincos' follow
peglgears.o: In function `T.48':
peglgears.c:(.text+0x854): undefined reference to `sincosf'
peglgears.c:(.text+0x88c): undefined reference to `sincos'
peglgears.c:(.text+0x91f): undefined reference to `sincos'
peglgears.c:(.text+0xa2f): undefined reference to `sincos'
peglgears.c:(.text+0xabc): undefined reference to `sincos'
peglgears.c:(.text+0xb06): undefined reference to `sin'
peglgears.c:(.text+0xc59): undefined reference to `sincosf'
peglgears.c:(.text+0xc93): undefined reference to `sincos'
collect2: ld returned 1 exit status
make: [peglgears] Error 1 (ignored)
gcc -march=x86-64 -mtune=generic -O2 -pipe -Wall -Wmissing-prototypes -std=c99 -ffast-math -fvisibility=hidden -fno-strict-aliasing  -fPIC  -DUSE_X86_64_ASM -D_GNU_SOURCE -DPTHREADS -DHAVE_POSIX_MEMALIGN -DUSE_XCB -DGLX_USE_TLS -DPTHREADS -DUSE_EXTERNAL_DXTN_LIB=1 -DIN_DRI_DRIVER -DGLX_DIRECT_RENDERING -DGLX_INDIRECT_RENDERING -DHAVE_ALIAS -DHAVE_XEXTPROTO_71 -Wl,--hash-style=gnu -Wl,--as-needed -o xeglgears xeglgears.o -L../../lib -lEGL -lGL -lX11
xeglgears.o: In function `T.84':
xeglgears.c:(.text+0x273): undefined reference to `sincos'
xeglgears.c:(.text+0x34a): undefined reference to `sincos'
xeglgears.c:(.text+0x402): undefined reference to `sincos'
xeglgears.c:(.text+0x45b): undefined reference to `sincos'
xeglgears.c:(.text+0x4b4): undefined reference to `sincos'
xeglgears.o:xeglgears.c:(.text+0x50a): more undefined references to `sincos' follow
xeglgears.o: In function `T.84':
xeglgears.c:(.text+0x944): undefined reference to `sincosf'
xeglgears.c:(.text+0x97c): undefined reference to `sincos'
xeglgears.c:(.text+0xa0f): undefined reference to `sincos'
xeglgears.c:(.text+0xb1f): undefined reference to `sincos'
xeglgears.c:(.text+0xbac): undefined reference to `sincos'
xeglgears.c:(.text+0xbf6): undefined reference to `sin'
xeglgears.c:(.text+0xd49): undefined reference to `sincosf'
xeglgears.c:(.text+0xd83): undefined reference to `sincos'
collect2: ld returned 1 exit status
make: [xeglgears] Error 1 (ignored)
gcc -march=x86-64 -mtune=generic -O2 -pipe -Wall -Wmissing-prototypes -std=c99 -ffast-math -fvisibility=hidden -fno-strict-aliasing  -fPIC  -DUSE_X86_64_ASM -D_GNU_SOURCE -DPTHREADS -DHAVE_POSIX_MEMALIGN -DUSE_XCB -DGLX_USE_TLS -DPTHREADS -DUSE_EXTERNAL_DXTN_LIB=1 -DIN_DRI_DRIVER -DGLX_DIRECT_RENDERING -DGLX_INDIRECT_RENDERING -DHAVE_ALIAS -DHAVE_XEXTPROTO_71 -Wl,--hash-style=gnu -Wl,--as-needed -o xeglthreads xeglthreads.o -L../../lib -lEGL -lGL -lX11
xeglthreads.o: In function `MakeNewTexture':
xeglthreads.c:(.text+0xee2): undefined reference to `cos'
collect2: ld returned 1 exit status
make: [xeglthreads] Error 1 (ignored)

Patch committed.  Thanks.

I guess my gcc replaces trigonometric functions by something built-in.  I didn't see the errors and nm did not list any of the trigonometric functions.